3impact for instance I think doesn't allow stray traingles or vertices. It's difficult trying to model as it is whithout having to try and make everything a quad.
Well I haven't used 3Impact in a while, it's true, and it is a pretty ghastly little engine for its peculiarities and arbitrary rules ( not to mention some out and out lies ) but the only issue I'm aware of with it is that you can't have unwelded objects if you want to cast shadows, because their engine makes no attempt to fix stencil volumes built from open geometry. That wouldn't have anything to do with quads though. You could just as easily have an unwelded quad as an unwelded triangle. If that's not the issue you're referring to, you'd have to link me to more details on it for me to be able to say anything further.