Until very recently I mostly played my main and my main only, and thus, ICly, just my main too. I tended though to build a multi layered faceted character with more than just one faction. The true inner faction (which is SoCT), the apparent idealistic faction (CONCORD) and the core background faction (Ammatar/Amarr). Since we all play freelancers it's not that hard to have an employer or a loyalty inducing faction, and a background faction that can be different. And even other factions to which the character can have heavy sympathies, like mine have for SoE or CONCORD (though CONCORD is slowly decaying to no end considering how ludicrously that faction has behaved since TEA, and this more and more). I am pretty sure that a lot of other characters come from a background that can be rather different from where they are now, like a born Intaki working for Mordu, or whatever.
Now I play secondary characters that I use more as "NPCs" like a GM would, to enrich the world as much as I can. I'm a vet after all, so PF has stopped to be an obstacle for me. They do not ask the same time to devote and can be used to reply on the IGS, or just come say hello on IC channels... And eventually they can develop enough to become full fledged characters (though i'm not going to do it since i'm not playing much now).