Snipers aren't strictly necessary, but they make things, much, much easier and less risky; again the most powerful/impactful class in the game. For that reason, I prefer to build around them in Ironman Impossible, and Heavies provide the natural cover clearing synergy and firepower to back theirs up.
My thoughts on desirable DLC:
http://forums.2kgames.com/showthread...h-DLC-Wishlist