Ask yourself this question: what fraction of businesses that want to be ad-supported actually turn enough of a profit to be worthwhile? If the percentage is high enough for you to consider it a reasonable risk then go for it.

[edit] I can't imagine starting a business unless I knew exactly who would buy the product, why they would do so, and I had a fairly good idea of how much they would pay for it. That means I start by figuring out what people need and then examining if I can provide it. But I'm not a millionaire yet.

If you just throw something against the wall to see what sticks, something will. The question is will it be enough to be a worthwhile use of your time?