This obviously changed with the specific group. When we played the "accepted" model was a bit like the "stable" system several folks used way back for D&D play; every player had one wizard, one companion and a few grogs in the campaign, but you only played one of them (or the "squad" of grogs) at any one time. Play tended to proceed in "missions" (or "quests" or "situations"...) and you took what seemed the most appropriate of your characters for the specific task at hand. Often the beginning of the "mission" took the form of everyone playing their wizard, discussing and deciding what to do - thus, each player could make the case for their wizard or their companion going along. Since wizards tended to have "downtime" stuff to do that was at least as valuable as "adventuring" it generally worked out well, since not everyone wanted their magus to be out of the lab for several days.
To be honest, I don't remember where we picked up this method of play - whether it was something in the rulebook current at that time (AM rivals D&D for number of "editions", although they are all very similar in actual rules), or whether it was something we picked up from other games and general "buzz".