This is probably the biggest part of the answer. You won't get anywhere (excepting the anomalous entrepreneur or "gift horse" factor) without a diploma. You need a diploma in something (preferably in your work field) to be on a level playing field with others trying to do the same thing. It is a GREAT benefit but it also allows you to not hit the ground running behind the rest of the pack. It's simply a must nowadays.
It's also the easiest way to climb up the ladder in the corporate World as you usually get to start halfway up.
My opinion (and this is coming from someone in the corporate World) - Formal education is simply the easiest way to climb the corporate ladder (by a HUGE margin). I've never stepped on anyone and I'm less than 5 years in this environment and I've already skipped many stepping stones (mind you, I'm also very prideful of my work and do the best I am able - this DOES make a difference as well. I don't have any severe job-changing, cut-throat experiences like you see on TV).