Though I am no expert what I have gotten from my readings, is that you really want the ideal pitch. I like to think of it as an orgy!
Where your yeast multiply enough to produce some essential products only achieved when yeast reproduce, that would not be achieved if you over pitched and the yeast just started eating (in which case you would not need the oxygen). Your yeast however would be glutinous bastards that did not work for their food. The yeast should reproduce, but not get sweaty and dirty! You kind of want them to have a quickie get the munchies and go for a mid-night snack!
I have tried this with a two gallon batch one I over pitch and one I pitched the recommended amount, and I could definitely tell the difference between the two, but only when they were side by side. If you under pitch then the yeast will be stressed out and in these I could tell the difference immediately. Since I have been pitching correctly, my beer has improved tremendously.
Oxygen is vital if you are pitching the correct amount from what I have read.