We have a Tremere in our covenant...he's been a pretty steadfast member. The part to remember is that there is a character underneath the House affiliation, and that personality is going to drive motivation as much-- if not more than-- the House. Magi are individuals first, and there are plenty of instances where the individual might disagree with the path chosen by the House. Everything depends on the character's desire to advance within the House, if they can reconcile the House needs and their own, if they're willing to accept consequences when they can't.
Honestly, the ArM5 Tremere are political, but they're a fraternity, a sort of Hermetic Legion. They might have policy goals they'd like implemented as a House, but they're also a large group of people who will have diverse personalities. That will influence how the policy is implemented more than anything.
And for the disbelief of many player covenants, I think many spring covenants are formed by a mutual need for survival and independence. When you start your own covenant with a group of freshly gauntlet'd magi, you get a certain level of equality that doesn't exist otherwise. New magi might then approach that covenant as time progresses specifically for that reputation of equality. Everyone brings something to the table because together you have strength to pursue your interests without fear of mundane reprisal. You can establish your personal projects, investigate what you like and know that there is a division of responsibility that will help protect you from the big big world.
As time progresses, yes, certainly more powerful magi have the means to strike out on their own, to be more discerning regarding the inclusion of covenant members, but then there are the thoughts of politics, legacy, reputation. Few like the idea of seeing what they've built crumble, and over time, maintaining that requires new blood. While some might find being an eremite a pleasant release from the responsibilities of a covenant, I think it probably brings its own series of complications.
-Ben.