There are, I think, two elements to this-
The other thing I keep having images of is ancient (like ancient Greece) naval battles which consisted primarily of ramming two ships into each other until one of them breaks.
If we go by this (and I'm not opposed to this), anything crossing a ring/circle, breaks it.
Yes, that explicitly includes a blade of grass falling across the line.
That is a traditional way of symbolically 'cutting' a magic circle.
How much it takes to break a circle is very much YSMV and the exact amount is in general better not hard defined. Some groups like playing with 'a speck of sand is enough' and others go to the opposite 'the material of the circle must be physically broken'.
While I have never played in a game that went with the speck of sand approach, I have played in ones that range from just covering the width of the circle in one area is enough all the way through having to actually destroy enough of the circle so that there is a gap in it (no matter how small).
Any change to core after this long a time that sets exactly how much is required to break a circle will break many games and cause an explosion of HR to allow groups to continue playing the way they prefer rather than meaningfully clarifying things.
I would support the blade of grass provided it was done by intent rather than by accident, or if it happened after the passage of a year (for the classic tale of a demon trapped in a circle until some freak occurrence broke the circle after a very long time)
And that is perfectly alright if that is how your group wants to play it. However it is not how every group wants to.
I am just pushing that it should be something that should be up to individual groups rather than some errata that most will never see or use. How to handle what breaks a circle should be up to each group.
I would tend to concur, but if so, it should probably be made explicit that this is saga dependent, and that all existing material is compatible with the entire spectrum of "rigour". I.e. you should explicitly define it as undefined 
Indipendently, I also think that one extreme aspect should be made explicit: that crossing the Circle does not necessarily break it.
My current group is middle of the road. We require that something completely cover the width of circle to "break" it. Also physical damage to a section of the circle can "break" it, though exactly how much was not defined to give the SG some degree of freedom.
So for example in my group, a speck of sand would not "break" a circle. A blade of grass also would not, unless it was large enough and landed exactly where it covered the drawn width of the circle. Circles with a thicker "body" are harder to accidentally break in our game. We also allow people to walk in and out of the circle without breaking it (as long as they are not warded by it and do not step on it).
Our version lends a flavor that our group likes (or at least does not care about). If our current game were to end and we started a new one, we might (Ok, most likely would) play with a totally different way to break a circle.
I think the more fundamental question is the degree to which magic is ruled by symbolism vs. intent vs. absolute rules.
I'm not sure this is absolutely true. I mean, is there more story potential in saying that the Aegis only works on Durenmar, and the characters are trying to replicate it on their covenant?
The question is whether you want your stories to focus on achieving that effect (in which case, yes, there's obviously more story potential) or whether you want to use that effect to enable stories focusing on other stuff (in which case there's obviously less story potential).
See now why usque ad sidera usque ad infera circles are useful ?
More seriously, I agree with the poor aesthetics. But I do not believe they belong to Wards as "paralyzation attacks".
They belong to mobile Circles used as hooplas to carry effects. Those are ugly (when they are not comical: I can see a Flambeau wanting too show of his mastery of both the school of the Founder and that of Vilano tossing hooplas-of-flame over enemies).
Wards used to entrap enemies are ok. I mean, trick a demon into an iron box, and ward that. Even ... create a big iron box around the demon, and then ward that, is not bad.
I would argue that this is also Saga dependent.
Though I would think 'absolute rules' are the least supported, since no two Magi learn or use the same spell. Magus A's version of a spell is different from Magus B's version. That and a few other points make me personally learn towards 'intent'. Support for 'symbolism' shows up most clearly in the mystery cults.
I think there has to be a balance between what is determined and what is decided by saga- realistically the question of symbolism vs absolute rules exists in fundamental aspects of the game, like shape and material bonuses, and is an underlying philosophy of how those rules are written. You can't leave 8everything* to the saga or you have no rules for the game.
I feel that looking at something and deciding if 'hard setting' it would improve the majority of games verse reducing the possibilities is what should be looked at before making new hard rulings.
If setting something is not required to make the rules work, then leaving it open to individual groups provides a wider range of options. I do not think hard setting which of symbolism vs. intent vs. absolute rules is required to make the rules work, which is why I lean towards it being saga dependent.
EDIT: There are many things I believe should be hard set and have even contributed some to this and the other errata thread. Currently this is just not one of them. If you have some argument for why it needs to be set then you might change my mind.
fundamentally, for consistency. I don't think it needs to be defined by the rules, but I think it needs to underlie them so the rules have the same feel to them instead of one rule being written based on one set of principles and another rule being written from another set. On the other hand a lot of the appeal of ars magica seems to come from its inconsistency, so that becomes an even deeper level of consideration...
You know, just state that Aegis are not usually mobile, but some troupe might want to allow it for ship-based convenant. That's enough to let players develop their own take on movable Aegis and realize what impacts it has on their universe.
I would generally agree that if we're just doing errata and clarification, rather than trying to build a defined long-term balance an redoing everything cascading down the line...
Then the best answer is probably going to be a sidebar reading
"Aegis of the Hearth, and other T:Boundary spells, are not traditionally mobile, although you may wish to consider allowing such if you want mobile covenants to be able to carry an Aegis of the Hearth."
Similarly, its very easy to say room and structure can move (for a wagon or a ship) but boundaries do not traditionally move. If you have the edge of your covenant dictated by massive pylons of stone, moving those pylons of stone does not extend your aegis further.
The golden rule is very nice for games that consists only of mechanics, some of which are optional. But when that game is heavily dependent on the lore and setting, such as Ars Magicka is, while house rules are always possible, the more you add flip a coin into the main body of rules, the more you end up with tribunal books having to write stuff like "This covenant may or may not have an aegis" and "this character may or may not work as written". The alternative to doing that is that the optional rule isn't supported anywhere, and contradicted by every material published, so suggesting the optional rule was probably not useful except as a story hook for what if original research created this breakthrough. I don't think any experienced storyteller is unaware they can't modify content if they want to. But common rules should be common rules as they enable line coherence. Then home troupes can decide whether they want an additional mystery cult, Ex Misc lineage and whatnot.
It isn't absolutely true in all cases, but ship covenants are unusual. If you want to play with the stories that come from having an Aegis, play with a stationary covenant. If you want to play with the stories that come from trying to get a functioning Aegis, play with a mobile covenant. (Or House Rule it.)
I think I should make a proposal about this, in a new thread.