I'll admit to switching sides purely out of self-interest.  
I look at two things when I log on.  One, who has the lowest numbers.  I switch to the side with the lowest numbers before I even bother to lift.  Second, I look for good fights.  If I find I can't find any decent fights on the side with the lowest numbers (rare, but it does happen) I switch to the side with the second-lowest numbers as soon as my hour is up.  It isn't at all abnormal for me to fly for two different teams the same night.
I can't say I do it for easy perks.  I have so many I really don't need any more.  I can't say I do it for low-cost perk planes either.  Again, I have enough perks to fly any perk plane I want, any time I want, regardless of cost (my favorite is free, anyway).  And I can't say I do it due to losing the ability to fly the plane I want, because I seldom see that (no doubt due to flying for the lowest-number team).
I also can't say I do it because I want to help any team in any way, because I don't.  I couldn't care less what state the "war" is in, or who's winning or losing.  I'm only playing to shoot the red guys down.  No more, no less.
If I see any gain/advantage to switching as I do, it's simply that there are more targets to choose from, with far less of an issue finding multiple targets that I can have all to myself.  I don't like sharing.  Often as not, I'll break out of a fight if another friendly joins in, or fly right past a bad guy if I think a friendly will jump in before I can finish the fight.
So, yea, I switch because it suits my idea of fun, and it's all in my own interest.  Nobody elses...