For every person making $XXX a day with AdSense there are a thousand making $XXX a year. Or things are humming along smoothly until you wake up one morning and you've been smart priced back to the stone age and aren't making anything per click anymore. The only guaranteed winner with AdSense is Google themselves. The reason AdSense is so successful is because Google takes an unspecified cut of every click and it's an industry built on the back of ringtone scams and click fraud.

Being beholden to a market that consists of a bunch of AdWords account managing hooligans that gnash and claw all day to drive bid prices down and are being governed by Google isn't really the kind of market you want to be counting on long term. Which just goes back to my point that while things might be good one day, by relying on an outside party for revenue, you might wake up tomorrow and be screwed because of decisions made by people totally unrelated to you or your product.