I've never understood the concept of vouchers in the first place. If buying a voucher got you say £21 of value for every £20 you spent it would make sense. But all that happens if you in effect give one a £20 note along with a restriction that it can only be spent in one place. What makes that a good idea?

The fact that, with the HMV case and others, you may not be able to spend it at all makes the idea even odder