Keep your friends close, but your enemies closer. If you know a Pc is evil, would you rather have them off somewhere potentially DOING evil or right where you can keep an eye on them?
My view (not nessecarily the teams) on it is that questing with alignment opposed PCs is fine as long as you aren't aiding that Pc is doing something that is directly good or evil respectively.
Tl;Dr Aiding a Pc in doing something your alignment is opposed to is likely to get you a shift. Just doing mercenary work together likely won't.