To me is all about the personality I want to give the sim, it doesn't matter if the trait is beneficial or disadvantageous to the sim. In my favorite family (the one on my signature picture), everybody but the youngest boy has some "bad" traits: The father is a "loser" and "absent-minded", the oldest daughter is "clumsy" and "mean-spirited", the mother is "insane" and "kleptomaniac" and the youngest daughter is "grumpy". The mother is also a "slob", but this trait is more good than bad, as being able to ignore messy environments and lick plates to satiate the hunger is good.
"Clumsy" and "Absent-Minded" seem to be common on my families.
Sometimes, when the personality of the sim changes too much from what I planned with the traits, I cheat to change the traits.
