I cant count the number of times I've been on a UA flight where they've asked for volunteers due to overbooking, but the only thing they offer is their worthless "vouchers."
In many of those cases, had they offered cash on the barrel or a voucher good for guaranteed, R/T ticket to anywhere UA flies with no restrictions, I would've taken it.
But I have too many unused vouchers because they are worthless. In fact they are insulting. UA is counting on suckers rogering up for them.