Probably because they haven't started with them near the start of the game and had a chance to develop them themselves.
Or (like me) they find the charaters personality (their speach effects and other stuff) annoying.
So why not try playing with those potential party members you find annoying almost from the start of the game (go straight to them, doing hardly anything to get exp. so you can develop them) and see what they're like then.
And I still REALLY want to know what people have against Imoen, her theiving skills can be devloped the way the player wants so no more excuses of "she doesn't have a high enogh score in ..." (probably said in a whining Khalid voice)

I totally agree there, mate. ;)

Imoen although a good thief, cannot be a good archer and cetainly not a fighter!!! Therefor you get Coran, who although not as good at finding traps give him a bow, keep Imoen for find traps only possibly to inflict 5-10 damage with the a short bow (the only one she can use.) And Coran to do other thief skills!!!!

If you want to put your views forward on Imoen the see my "this Imoen thing" subject.