By my very rough count, 95% of the ArM5 core book deals with magi: creating magi, creating new spells and items, their running a covenant, their interactions with non-magi on those rare occasions the magi leave their covenant, etc., etc.
By contrast, companions very rarely and grogs never are able to use magic. There are only a couple pages used to describe creating companions and grogs. The adventuring mechanics--including combat--only take up less than a dozen pages and are relatively abstract.
The predominant focus of the book is on the magi and using them as the focus of your campaign. If you essentially aren't going to use magi I don't understand why you would use Ars Magica at all.
This is largely true, although the supplements have a roughly 1:3 balance (i.e. 1 companion to 3 wizard oriented splat books). Although I think a non-magus Ars Magica campaign can be very interesting, the system poorly supports it, and Ars Magica is all about playing the wizards.
However, the system also explicitly encourages the magi to (rarely) join adventures. Story flaws obligate you to do so when the SG draws that card, and he invariably has carrots and sticks to wave around. I don't find staying in the covenant to be a problem at all; it's just the character's down-time, much like a D&D PC's downtime between adventures. Having the wizards stay in the covenant is more a matter of troupe-style play and group choice. In normal D&D, the wizard doesn't stay in the tower in-game because he finds in-game reasons to leave his "studies"; the same can be true in ArM, if the "gaming contract" is like D&D - i.e. every player plays his (only) character in every adventure. The only difference is that ArM wizards actually benefit from studying in their tower - which I can't consider against it, it adds to verisimilitude (and it can be ameliorated or reversed at the hands of a good SG). The decision to stay in the covenant is a matter of play-style.
I don't find the overwhelming 80% book-keeping true at all. I think it depends a lot on familiarity with the rules, but in my experience adventuring sessions take up the vast bulk of the time and book-keeping advancement is done in between and does not really take that much more time than advancing a character in 4e, say. (The 4e rules are simpler, but most time is spent bickering with yourself what you want to do anyway.) Book-keeping the covenant and especially its grogs can be a bitch, but that's the DM's problem
Overall, I find ArM involves lots more book-keeping (perhaps indeed 20%), but most time messing with it is spent on the fun "what am I gonna do now?" question, rather than the dull "mark 7 in that column..." chore.
[Incidentally, I'm playing in that play-by-post, and we're playing magi. All the time is spent on adventure, but we haven't really gotten to the seasonal-stage yet, so we'll see.]