Google doing a YC is a pretty weird proposition, that's Goliath going to have breakfast on Davids dime, and asking him to help lifting a bunch of stuff to boot.
Of course YC is orders of magnitude larger than the companies they invest in, that's why it makes sense in the first place.
As to your other points:
1) I've seen little to no evidence of that, rather the opposite, even YC start-ups that would be non-starters or that already have an established competitor, sometimes with a better product that has gotten no coverage whatsoever are covered by anything from TC to CNN just because YC is on board
2) this may be true, but that also means the size of the pool of alumni with valuable knowledge is larger. I used to think that YC had a 'scaling issue' in the person of Paul Graham, but in fact the alumni seem to offset that in whole or in part
3) the marketing value is already worth it, anything you get out of it on top of that is pure cream.