Your comment is pretty spot on about the admission dynamics.
> Legacy
I think it is more of a tip than you are making it out to be simply due to yield farming - the smart kid who has a Harvard parent is more likely to go to Harvard over Yale than a generic smart kid, so if you want to keep your admission rates as low as possible you tip legacy.
> 10-20 kids/year who get in this way who wouldn't otherwise.
They certainly exist, Harvard has the z-list.