Well, being evil is still inconvienient... But you are right, evil guys are overpowered (well, there is no decent evil thief, only good and easy to get is Imoen, especially if you dual her to a mage, Safana is neutral IIRC).
Montaron can do everything, you don't need a pure thief. Imoen is annoying to no end, even though she can stay with the party even if you are a villain. Safana being neutral doesn't change much, the only time she'll leave is if you drop to 1 rep. Playing with an evil party gives the game a much different flavor. You are on a mission for your own betterment, you are powerful and you know it, you can do whatever you want and you take no shit from anyone. I wouldn't say it's darker per se, just more ...my type. Good parties are a little bit too happy-go-lucky, like a bunch of children playing on the playground, just going out and having fun adventuring together, slaying dragons like white knights. Both approaches are valid, it's just interesting how different the game can feel depending on that. I guess kudos are in order for Bioware.