Not to mention that exit valuation is a dubious metric of startup success to begin with. It also excludes startups that don't exit. Airbnb, Dropbox, etc wouldn't be included.
Of course, one can argue that startups should be not be in it for the exit, but when you take VC money, that is what you are signing up for.
Fred talks about he defines success here - http://www.avc.com/a_vc/2010/06/how-we-measure-success.html
"We are financial investors and we do want to see our portfolio companies become valuable."
Say the startups with the highest (or lowest) funding all die before exiting -- that's a really important thing to know. That'd affect the regression outcomes (not to mention the decision-making process of everyone involved.)