If you use sketchup to model certain elements in your animation that is fine. Just make sure you bring the models over to 3ds max and complete your animation in there. Creating the entire animation in Sketchup is a no no.
I look at it this way. If the Bulk of your work is done in 3ds max. Then its fine. Heck, if your a very good 2D animator and want to make a 2D animation and want to use 3ds max only to flip the frames using a series of planes, that should be fine. I've used 3ds max like a 2D animation program before You can get interesting
results.
Plus 3ds max is a bad at making textures. Especially if you have a character you want to paint the texture on him. Photoshop should be allowed.
and remember animation is about expressing emotion, You don't need complex environment or realistic environments to have good animation.