Yipes. Decreasing legal expenses is about 1% of the value of YC. By far the most important thing we do is work with people on their ideas. Then teaching how to present to investors, then introductions, technical advice, advice about negotiating deals, help in disputes between founders, the value of the other YC founders, help in recruiting...
Ditto. I misspoke. ("contributing motivator" would be more accurate than "_the_ main reason.")
Having legal counsel in the know is certainly just one of many pieces, it all depends on which ones you need (most). (And that in turns depends on what precisely you're going after.)