Something like 99% of Harvard end up graduating, so simply having a Harvard degree doesn't tell me much more about you than simply having an admission letter. If you were admitted, then it's almost certain that you'll get a degree.
So, if instead you hold onto your admission letter, as proof of ability, and then go to a lower tier school where you can be more certain to be one of the biggest fish in the pond, you get the best of both worlds. Hell, even include a list of "universities admitted to" on you resume to signal the brand name to employers that aren't smart enough to realize the brand doesn't matter much anymore.
Then again, why would you want to work for an employer that still puts stock in college brand names over individual ability?