Those cards do have cash value as you can buy them as gifts. I know this a as we had a huge falling out at work when I used mine.
Part of my bonus structure allowed us to win from a choice of gift cards. Tesco, HMV, Arcadia and a well know Pizza place. As we were planning a team night out we decided amongst ourselves that any of us who won would claim for the restaurant vouchers. As luck would have it I was on for £25 as was another member,and another member £50.
One member who won £25 knew she couldn't go as she was away but on the basis we were covering her authorised leave (she'd ran out of holiday) she wanted to put hers in 
That meant 10 of us going. The meal was £15 per head, so we'd only have to pay £5.
Then as luck would have it, one of my clients submitted a purchase order a month early - winning me that months top performers prize - a £100 voucher of my choice
my boss said I didn't have to contribute it toward the meal but I chose to.
We were thrilled, it meant we had £50 to spend on booze 
About a week before another team we worked with asked if they could join us. We said fine it was a meal, and then to local tacky nightclub. So 6 came.
When we ordered the meal we basically ordered £50 worth of house wine for the table, which was shared between all 16 of us.
Bill came. We put the vouchers down, my boss had said to the other boss during the meal we had agreed they only need pay for their meals, not booze. One member of the team handed a £25 voucher and £5 for him and his friend. So that left £60.00.
The other boss, I shit you not, pulled out her phone and said " bargain, so that's £4.50 per head including tip"
We were all 
My boss was 
We then agreed that our team would pay £1 each toward the tip but the remaining three needed to pay their £15 each. Happy they weren't, two of the girls through down £15 and walked out there and then. Other boss paid but sulked the rest of the evening.
There was bad feeling from the two girls and the other boss for months. I had to train one afterwards and it was impossible as she refused to speak to me. 
The fact that this was nine years ago and I am still, literally sat here cringing as I remember how horrible and uncomfortable it made the evening, shows I really out to get a grip and be over it now. But it was awful.
Their argument had been we should have used the vouchers to cover the meals and everyone pay for drinks.
Ours was it's our fucking money, be grateful we let you get pissed on it.