Doom beta is a PO incentive - which I'd assume is on a similar engine with similar quality, thus double dipping in goodness.
Besides even without it, I bet I'd forget if I'd not P/O'ed it - so much shit going on in my life that I'm sure I'd forget them if I didn't P/O [Got phones to replace in the next month, house in the next three, car just added to the list thanks to my accident today, and getting close to a dozen games already coming in the next two months already]
Also I did it as an Amazon P/O - so it's not even a dime down until it comes out - they'll charge me when it ships, literally out nothing if I change my mind or whatever.