I sometimes dont't like that celebrities advertise a project, they were usually paid to do so, and that does not reflect in the project itself, now if a celebrity advertise on its own, due to actually liking the project, then that is different, but any celebrity will do that, because they have a lot to gain by charging the project itself.
I agree, celebrity endorsements have no long term effect on the project because everybody knows its just a marketing strategy but I'm inclined to having experts endorsing a particular project conveying his own expertise on his chosen field of endeavor.