Originally Posted by
DrHalxx
Is it really revenue protection if the seat flights empty eventually?
Yes, absolutely in airline terms.
The real aim is to make you pay cold, hard cash to sit Business Class - these vouchers can be seen as a 'try before you buy' incentive to create that appetite, but it's why space is limited. If everyone believes they can upgrade for nothing because there are spare seats there, they'll never pay up. It may be why sometimes additional I space is created, and sometimes it isn't - if your history shows no Business Class purchases, they're more likely to grant a discretionary upgrade than if you regularly travel in the cabin but have chosen to chance your arm on a voucher.
It's very common amongst the European airlines - to quote a specific example, BA offers something called an AUP (Airport Upgrade Price). This is typically (at least on longhaul) less that the difference if the higher cabin had been purchased at time of booking. However, they do not always offer these, and if you take too many each year you're denied an offer, whereas the person who asks immediately after you is given a price.