You can use the tokens at anytime at either bar but must purchase the tokens BEFORE the game. The bar near the Tunnel Club ONLY accepts pre-purchased tokens at half time (no cash) that's why it's so empty/ quick to get served.
Last time we bought a beer each and 4 tokens each (max allowed) at £2.50pp. We used them at half time. We still have 3 left and was told they were valid all season.
There seems to be no con, just a great deal. I hope I've got all this right - it seemed rude to stare at a gift-horse's nashers !
Now living down South it wasn't the price that I was chuffed with - it was the lack of queue , so there was time to sup the pint.
Hope this helps.