This is true, though the evil characters such as Edwin, Korgan and (later) Sarevok choose to stick with the PC because they can see that s/he is on the way to great things, and thus it's not in their best interests to screw them over.
I think the main problem with the good-evil balance is that Baldur's Gate is essentially a story about triumphing over evil (not just evil enemies, but the innate evil within) rather than giving into it. Were it more flexible then it would have to allow for things like, say, screwing over the elves and helping Irenicus merge with the tree of life in exchange for your soul back, or somesuch.
It's pretty clear, though, that you are being steered towards the good path because of the allies you are offered are mostly the good guys (only the Shadow Thieves are morally grey, and even then they are the lesser of the two evils you are forced to choose between).
Though this is true, with the evil NPC's and all, the odds are that if they stick around long enough, they'll slit Charname's throat in the night, and claim the Throne themselves! After all, it's a mage-eat-fighter world out there, and only the strong survive- but I digress.
I would agree that the lack of "true" evil options in BG2 is appalling, but I can totally understand why they did it. Doesn't mean I'm not looking forward to the Mod for the Wicked (gogogo killing civilians and getting away with it!)....
-Argus