Excerpted from this post about third-party monetization with Yahoo! BOSS:
Our current policy is, while we do not prevent you from implementing a third party monetization method in conjunction with your use of BOSS, we may at any time require you, for example, to implement and display Yahoo!'s Sponsored Search (or similar) advertising offering instead of a third party's. Alternatively, we are considering the option to pay a fee based on your use of the service; in this case, we would anticipate permitting the continued use of monetization methods from third parties.Basically, Yahoo! reserves the right to fuck you up: any time, any place.
For small niche-like sites this is probably OK; they wouldn't bother trying to get any cheddar from you. If you run something moderately successful powered by BOSS you might have problems in the future.
I don't have a problem with having to display YPN ads or whatever. The real problem is not being able to support third-party monetization efforts. For instance, if you're running a site that makes good bread with CPA ads / Amazon affiliate links, removing those and replacing them with lesser YPN ads is a kick in your junk. What do you do then? Try to manage direct sales?
Something to think about.