Yes, potentially. But it's not limited to that. It also helps me to better design something, or as an excercise in lateral thinking.
Sometimes at the table we tend to derail to nitpicking, but it's always as something we are all enjoying. When someone gets bored we just have a codeword and everything stops, allowing the events to move on.
There is away around the familiar teleportation issue, that our troupe worked out. Build the spell with Target: Circle, and an Animal prerequisite. The prerequisite makes it a bit harder, but still doable.
This brings up another way to do it. Rework the spell, Leap of Homecoming, to specifically apply to teleportation to touching an individual to whom you have an arcane connection.
Marko, you are not in the minority.
The "TP to a familiar is fine" solution is played wildly. We don't need to discuss it more than it has been.
There are plenty of pro/contra arguments, and since it's a fictional world, with fictional rules, the final point is : do what is good.
Most players like it to work.
If a SG doesn't like it, he can make it fail. If his players are unhappy with that, they can say it and he can change back. If he doesn't and they are still unhappy with the bookkeeping (rock A (1248 - duration 4 years) from there (place changed, doesn't work, now go there), rock B (1225 - duration 10 years) from there, rock C (1238 - duration 7 years) from there...), they can quit and find another SG.
But I guess most players will just say: "I teleport to the place my familiar is in" and soon becomes: "I teleport to my familiar".
for my side, Ars Magica is more than other RPG a game where the DM can't have all powers.
I need players to participate into creating interactions, goals, story and other characters personality around the main saga.
It's especially true for magic, i want players to get their mage study magic theory to invent new spells and/or spells use, things i won't have though about directly.
And to do so, we need to share a common understanding of magic system.
So, not opening strange exception leading to loophole inside the system is a key here.
AC linked to touch is a loophole (provided examples already)
transitional AC is a loophole (didn't provide example, but it's even worst leading to link from anything to everything)
now if character wants to make such usages a key part of their character goal through breakthrough, then let's go
Another point is than Tp can kill saga, when wizard can inhabit alone on top of a protected mountain and tp to carry task all over the world, you kill a lot of saga component (travel, grogs usage etc...)
long distance tp are happily hard to design spells, and i certainly don't want a single leap of homecoming spell to be a single shortcut to a go anywhere instantly spell.
Been there. Our high powered saga ended up with grogs travelling in small parties. Generally flying themselves with enchanted items, but that is beside the point. The fact is that they carried small teleporting boxes that teleported small items to the covenant and rang a bell if they thought they needed a Magus. They wrapped the items in suitably colored ribbons to define what Magus they needed and sent small shields, wooden chairs or wands dependibg if they were needing combat, negotiation or investigation assistance. It meant that for our parties mages ende dupnstaying home, and were summoned by the away team when needed.
This also caused some issues and related adventures, but in general it ended up feeling we were playing marine corps and not Ars Magica.
In the end we have cranked all teleport spells 4 magnitudes upwards. Yup 20 levels. The only exception is portals. Now our mages need to be nearer the front lines.
Let me build on top of this, from my point of view.
I completely agree with you here. What I try to do is exactly trying to figure out why someting is valid without becoming a loophole. The nitpicking is just for pinpointing why in the specific case something could work (or not), and how it could trickle to other fields.
BTW, having said what I said above, I still do not see much loopholes to the examples by Ellone. A broom in a room is an AC to the room as long as it is there. If it remains for a long time there it will become a stornjger AC to it, but while it is there it is an AC to the room. Regarding the castle, yeah, no problem there either: In the same way the pencil in my hand is an AC to Planet Earth if I can add enough size magnitudes to the spell AND overcome the MR that it is likely to have with the spell I want to cast. that is not a real problem, Really, especially since said pen came with me to the office, so the moment I step out of the building it will still be a link to planet earth (but not the office anymore). That is not a loophole, it is a feature of how touch AC work. 
Chain-linking does not work. if I have an AC to my familiar, I will be able to use it affect what he is touching. Mythic Europe for sure. and Paris as well if he is in a building there. But not London. Still needing to add a lot of size magnitudes.
In our game, there was a Necromancer in a neighbouring Covenant who got around this by wearing nothing but human leather and using bone tools.
He was accidentally killed and only some of his Coven-mates were really mad about it...
Edit: since there seems to be interest, my PC accidentally killed the necromancer. The neighbouring Covenant was attacked by evil Guernicus and his band, we flew in after the Aegis had already been blown up, to find out that the covenfolk were fighting a horde of zombies. My PC just used his Arc range attack spell on whoever was sending the zombies to kill the Covenfolk and dealt massive damage... This was a massive issue to explain at the following Tribunal.
I've heard about nailed down chamberpots on these forums, but never before about toilet paper ruining games... Then I figured out what was meant by "tp" a moment later. Yes, that's always been a worry for me. I generally find there are three things that can ruin fantasy games in general: healing being too easy (you end up fearing nothing but death, and maybe not even that), travel being too easy (you skip the adventure for little bits later on), and mind reading (you skip the adventure because you know everything). Ars Magica handles the healing issue quite well with penalties to learning and spending vis to heal. I personally handle mind reading by requiring understanding of a language to understand interpreted thoughts and the like, which is reasonable and makes it harder. Instant travel can still be a problem.
Wow this topic got really busy while I was away.
First, Marko, you probably aren't in the minority. I think most people made up their mind and didn't keep debating the issue. The first half or first third of this discussion, I was the minority thinking it shouldn't work.
I mean, I find the answer very simple; You can be a "lazy DM" and let your players use edge cases to their advantage, and skip half the stories about how people actually get to places with one of the most common spells in the order. Or you can be a rational human being that understands the joys in people using clean rules, and want a consistent feel to the beautifully designed magic system which uses hermetic theories and experimentation to test and find the boundaries and definitions of what magic can do. I honestly can't see why anyone would find merit in allowing this.
Joke aside, I admit I always tend towards a restrictive limitation on what can or cannot be done. I did find your comment to be a bit dismissive of people who disagree with you, a bit of a straw man argument. Anyway, my players know I'm cautious about allowing things to get out of hand, and I know this as well. If they feel I'm making a wrong call, we discuss it and find a group consensus.
I don't feel that a bird in a room is an AC to the room - not unless the bird has been there for a long time. I always think of the worst example of interpretations because I'm cautious of abuse - I admit that's a flaw of mine. If you get shot by an arrow, do you have an AC to the archer? He had it in his quiver for a day or two at least. I would argue not; The 'Days' AC uses 'a frequently used tool', which to me implies the apple I picked up and threw at you shouldn't be an AC to me. A 'favorite tool' is a 'Months' AC, so I feel there's a level of scale and mystical connection that isn't formed by 'my crow familiar flew into the courtyard, AC to there now.' My opinion of course, I just prefer the stories this way.
Clarifying: I don't think Marko's opinion is wrong, or bad (other than his belief that those of us having fun with discussing rules are Killer DMs who don't like fun). If I was a player in his saga, I would accept his rulings and have fun with them, because I think these decisions should be troupe and saga specific. I am a firm believer that adding a few limitations ot a game encourages creativity to deal with the problems.
I like the iron in your words. 
I need the iron in my words to properly kill my players, sir.