I agree, my (only so far) Maga has a passion for anthropology, and wishes to find out about exotic places like Hindustan and maybe even find Prester John. When searching for a familiar she found a curious cat (Might 8) who shared her passion for stories. The plan was that they would travel a lot soaking up the various Area Lores. The RoM:P chapter totally kiboshed that as there is not enough Vis to waste on that sort of thing.
Bob
Hmm, no, I disagree. The way I read the sentence is this: you can generally assume that a familiar knows the facts that the maga knows, unless the maga is particularly secretive and keeps the knowledge to herself. Note that this is not redundant. In general, a maga's grogs can't be assumed to know what the maga knows. For example, they can't be assumed to know where a particular poultice can be found in her lab, or what type of vis is held in the strange feather necklace she carries. What the text is telling us is that you can assume that the familiar does know this stuff, unless the player explicitly mentioned that the maga withheld the knowledge.
Why should that be a problem? A familiar does learn as a human according to the RAW, even taking RoP:M into consideration. A familiar with no Might learns exactly as a mundane human. A magic familiar with Magic Might 20 learns exactly as a magic human with Magic Might 20. I read the text as telling me that the familiar learns as a human, rather than as an animal.
Two unimportant remarks first.
a) keep in mind that when the magus is inventing spells, enchanting magic items etc. both magus and familiar are gaining exposure experience and that's it.
b) keep in mind that a magus will spend experience in many areas where the familiar just cannot spend it - for example, the Hermetic Arts - slowing down the magus relative to the familiar in those areas where they can both spend experience.
Still, it's generally true that a high Might familiar will progress somewhat slower than "his" magus under these rules. But that does not contradict the text, even assuming that "knowledge" here should be read as ability experience. The core book says: "Over the years, your familiar learns what you know". So a familiar could learn on year 2X what the magus learnt on year X. E.g. by the time the magus has Magic Theory 10, the familiar has Magic Theory 7. Over the years, the familiar will reach Magic Theory 10, but by that time the magus has Magic Theory 14 etc.
Which is why my HR doesn't put a kibosh on it. I want familiars to be interesting, my hope is that another player would be willing to pick up the familiar as a PC when the magus goes adventuring. My experience is that usually isn't the case, so by and large the familiar is a pile of numbers, most importantly intelligence and magic theory. They can be so much more, and if they are doing something with their magus, the might penalty from learning doesn't apply...
My experience is the same. Other players do not like very much playing the familiar of a magus other than their own. But I do not think it's because familiars are uninteresting - I think it's because they are too "tied" to the magus.
On the one hand, from my experience the players of magi are extremely protective of their familiars, almost jealous; they generally do not like other players to play them (there have been exceptions, of course, but "familiar-as-extension-of-the-magus" seems the norm).
On the other hand, a magus' player has a lot of influence on what type of character the familiar is - both mechanically (after all, it's the magus who decides which powers to instill in the bond, which magical items to give the familiar etc.) and narratively (it's the player who has most input on the quirks that the bond creates, and when the time come to acquire a familiar the storyguide generally plays along with ideas like "I'd really like a familiar who felt jealous about my magus' philandering!"). The result is that the familiar is in many ways much more the creation of the magus' player than of any other player - and in my experience (again, with some notable exceptions) players tend to prefer playing the characters they have created.
Yeah, I think there are a number of factors at play. There's the creation angle you covered, but there's also the I don't want to break it angle that players will have with such a character, too. The familiar, unless there's a lot of discussion about it and it is in the familiar's character, isn't going to be doing the crazy things that grogs will do. If it's a choice between playing a grog and another magus's familiar, I don't have to worry about breaking the grog, not the way I do with the familiar. As a player, I'd feel really guilty if there was a combat scene and I rolled a botch on a defense roll for a familiar or something.
Yes, that's what I meant when I said that players are extremely protective of their magus' familiar: "breaking" it comes too close to "breaking" the magus, so my players don't want other players to play "their" familiar. It's just that your troupe is probably nicer than mine: your troupe worries about breaking the toys of others, my troupe worries about their toys getting broken by others 
Which one? 
I am perhaps putting my spin on it more than anyone else's. I would hate to be handed a character and then have the fickle dice putting me in the position of having it die under my watch. But now I have an idea...
I've found the same issues, and I'm fine with them. As a player, I tend to want to play my magus's familiar, too.
The exception for me has been for companion-level familiars that were companions belonging to players beforehand. For example, I've designed a magical animal that desires to become a familiar to a magus (solid reasons in the back story). Ideally this animal can convince a magus to make the bond. The return for the magus is manifold: saving time, getting a really capable familiar, etc. But this comes at a loss since I'd be controlling the familiar so the magus's player has less control. I figure it's a fair trade-off between them.
But in both cases I want the familiars to be able to grow into the roles they have acquired as individual familiars, not just as generic lab assistants. So I like it when familiars are able to learn and grow.
Yeah, I guess that is a valid way of reading it. It makes the sentence much less important, since a player can essentially override it by make a statement as to what they expect their familiar to know, but at the same time, reading it as having a minor impact in the game rather than a major one does validate why there isn't any concrete mechanics attached to it.
That's a bit of a stretch. I think we can probably assume they meant a mundane human, rather than a concept that hadn't yet been mentioned until Realms of Power: Magic was written, but as I said before, it is an internally consistent reading and I can't find real fault in it.
On the whole I like your suggestion for the rule. I prefer mine because I think the rate of learning is more to my taste, but that is entirely a matter of personal preference and opinion about balance, so I don't think we differ on anything important.
I'm also firmly have a preference for playing my own familiar. Given the option between having a grog-level familiar that I play, or a companion-level familiar that a fellow player plays (I think those are the example choices in RoP:M?), I'd think I'd almost always pick the first.
A familiar's importance probably depends on the particular character it's attached to - to some I'd think it would be very high, and others might be only slightly more relevant than lab assistants - but so far, when I do play a character with a familiar, I like to use it to define who my mage character is. Either by having the familiar bond change the character, or by highlighting personality traits in opposition to the familiar.