The decision not to accept gift cards has been made by the administrators, not HMV. They're trying to find a way for the business to continue so need to do whatever they can to make this a smooth process. They desperately need to raise money which is why the stores are still trading, but unfortunately means they won't be accepting gift cards.
With any luck, HMV may be able to continue trading (in a reduced number of stores I suspect) and may even honour existing gift cards. The same thing happened with Game last year, but the new owners decided to honour gift and loyalty cards when they came out of administration.
If HMV do go completely, you may be able to make a claim against the administrators for your credit, but I can imagine staff wages and suppliers will be addressed as their main priority.
Like I said, they will only upset customers by refusing gift cards and it seems a very opportune time to block them i.e. right after Christmas when they must have sold thousands easily even with their flaky sales. Free money for them effectively as they get to sell a load of gift vouchers and now are getting away without honouring them. Blatant daylight theft IMO.
The reason I am equally annoyed with HMV as the administrator is because they made no attempt to warn customers in advance that they were in trouble or would enter administration (so people could use any outstanding gift card balance quickly and to discourage people to buy them for others as gifts which have become worthless in mere weeks), they denied administration right up till the end saying the blue cross sale was "perfectly normal for the time of year" thus giving customers no warning of their position and neither have they (yet) given customers any window to use remaining vouchers or gift cards before the shops go altogether.
Both parties are equally despicable IMHO.