I agree there is little understanding by some of how Adsense "works" and how $ from an advertiser ends up in a check to you from Google. Most people don't care however anyone running a business off of Adsense should know in detail how they work or you risk getting your business cut off like this. As others have said diversification of revenue is important especially if you don't have confidence in how the system works - example the stock market.

If you are releasing one or a few videos, revenue from Adsense is just about the only game in town. However if you have enough content that can survive outside of YouTube you create an opportunity to work directly with advertisers. Many video podcasters have moved this way, not hosting videos on Youtube and owning the ad relationships and revenue.