Agree completely. At the margin, $5m won't make a difference, just like at the margin donating $10 to cancer research won't make a difference. But that $5m check along with many other checks does make a difference - both in helping these school achieve even higher levels of excellence and in their ability to fund poor and middle class student tuition.
There are other valid reasons to donate to a university, but the really big names aren't going to start recruiting more poor kids if they only had a little more money.