"DO NOT RELY on ADSENSE as a sole revenue provider for your startup! Do not! "

This is really a classic business issue. There is always a tradeoff between putting all your eggs in one basket (say what zynga did with facebook initially) and having a diverse customer base. Same goes for relying on a single vendor or source.

Obviously if you can diversify it makes sense. But it all depends on the revenue hit your take by doing so. Extra money that you earn can be banked toward the future as well.