You are forgetting about one of the additional benefits of the voucher, you get a break on the tax paid. I had a $293 ticket go down to something like $252 of vouchercash.

So yes, this is something American does VERY right, issuing the voucher for bumps, so you can buy upgrades or tickets with them. With advance purchase tickets you can make a $400 voucher go very far!

The United vouchers, you pay the tax, and if it is a $300 voucher, and buy a $200 ticket you do not get the residual value in another voucher... your $200 ticket just became a $300 ticket. However now United gives you a restricted domestic ticket that doesn't earn miles. Could be worth more than a $300 voucher for a longer trip, but you don't get miles for it, and even for a short trip if the fare basis code isn't available, you can't use it. On American you could even buy a First Class ticket at the last minute, although you are limited by the dollar amount of the voucher.

Personally I like the dollar amount method with residual value refunded in another certificate. It gives you a better idea as to what you are getting when you decide to take the bump.

As far as miles, I got a 1,000 mile bonus in addition to the $200 certificate for taking a flight an hour later. $200 of American script and 1,000 miles was well worth an hour at DFW to me.