Well, ideally, I would want someone who has energy and ambition to succeed (succeed through the covenant's success, I mean). Suppose the covenant gets in trouble and disrepute: will the Tremere house support the covenant or will they simply order their magus to leave and find a more convenient covenant?
It depends on the kind of game you want to play, but I would say that Jerbiton and Flambeau are extremely practical for a covenant. Whom should we send when we need to deal with those pesky mundanes? The Jerbiton, of course (OK, maybe the Flambeau, if they are really pesky ). And whom would you like to have by your side in a fight? A Flambeau, of course. Yes, a Tremere army would be even better, but they are unlikely to mobilize to fight your covenant's fights.
But, for me, it's more a matter of commitment. OK, so you can't use the Mystery cult magic... but your sodalis can, and he is stronger because of that, and that's good for you too, as long as he is committed to the covenant. On the other hand, if the Mystery cult magus is only interested in his cult, I would agree with you. Why doesn't he go to live with his chums, anyway?
Yes, the obligations of a Mercere are certainly onerous, but then I don't see many players choosing one as their magus character, and ungifted merceres would take a "companion slot" in my saga. Besides, they don't require a lab and they certainly are an asset to the covenant in terms of information and contacts. In fact, as storyteller I probably would not allow a redcap in a Spring covenant unless there's a good reason for him to live there (maybe the covenant's placement is somehow strategic).
Yes, I agree. But then my question is: why would a covenant accept an uncooperative, unmotivated magus? My problem with the Tremere is that I can't see one of them caring about the covenant. Would they make sacrifices or even risk their lives for it?
A group of magi like the one Xavi described can be fun to roleplay (maybe they ended up together because no one else wanted them). However, if your covenant's growth is one of the themes of the saga, I would like sodales more capable of cooperation. In the sagas I have played the magi were cooperative (maybe with one or two black sheep), but why would a blatantly uncooperative magus be accepted in a covenant, unless all the other magi are like him?
Any magi can be uncooperative and/or unsuitable for your covenant, the problem is that I can't see any Tremere regarding the covenant as a real priority.