Larian have stated multiple times that more companions are coming. During EA, too. They've also said that the current companions are leaning towards evil, and that they included those evil ones first because players default to good. Some of the sources: Panel From Hell, yesterday's interview with Swen. Adam Smith has also mentioned that he's making a character and is eager to share it with us, but it's not in the game yet.

I like the idea of P:T-like dynamic alignment, but it has the potential to be very jarring and frustrating depending on how you roleplay. A Neutral Evil character could be nice 95% of the time because of pragmatism. The game will then "think" you're a goody-two-shoes. It could only work really well if the game knew the motivation behind your every choice and at this point it's just better to make decision-based tags that don't delve into morality (which I believe is how they handled it).