Most frats and sororities are legit. Those that aren't are the types you see stereotyped in almost any teen flick or what have you. On my campus, every single fraternity and sorority were legit. On other campuses I have visted, I have yet to see one that exists soley to party and cause havokchylde.
You are incorrect. Greek dues cover private parties, house repairs, expenses involved with philanthropy, which believe it or not, can run pretty high. This notion that greeks have to "buy their friends" is not even close to being correct.
Greeks provide more to the communities they are involved in then anyone outside the organizations will ever know. Just as one example, at my college we had a Relay for Life fundraiser. The school raised approximately $16,000. $10,000 of the money raised came from greek organizations. Every greek organization (save maybe 1 or 2) stayed up all night for the walk.
This is just one example of the many services, drives, and fundraisers that greek organizations do. While I'm sure some organizations exist soley to get drunk and get laid, the vast majority don't. There is far, far, far more to greek life than what is shown on television and in movies.