Disadvantage of said system for Google would be the fact that if you do that, people can skip ads much faster and they won't be able to do any tracking of interaction at all. For advertiser's point of view, that would be just worse version of TV commercial.
If so, they should pay for Q/A and/or focus testing themselves. Not freeload off from forcing users.
I can already see that this won't gather them any data that is actually useful for analysis.