When I usually play the sims I get bored with just one sim.... so... I like creating families with 3 sims who are slobs, couch potatoes, party animals, evil, etc. and have one 'perfect' sim that is the exact opposite. Oh, the fun.
Buuutttt.... on the other hand, when I start dynasties and such, I totally make a perfect founder.
And when I, em, 'experiment' with fire, electricty, and the like, making 8 coward unlucky insane neurotic loner sims with tiger makeup and the stupidest outfits? That's just too irresistable, my friends.