We always link a cc to our room card, and it works really well. Like everyone else has said, it saves taking money around with you, and you can get print-outs to check the purchases whenever you want.
We have done this 4 times now, and never had any issues with it being confused with Dining Plan, or any other problem.
The only tiny annoyance is that there is a limit of $500 you can charge before you have to go to your resort reception (or possibly to a park guest services, but I'm not sure about that) and clear the balance off.
All that this means is they give you a print out of the charges you've made, you agree it's correct, they charge that $500 (or whatever) to your credit card and then your room key can be used to charge again with the balance back at zero. Once or twice we have been caught out at a restaurant or shop when our room key was at the limit like this and had to use a different card to pay with.
Looking at it another way, it means that the most anyone could ever use a stolen room key for is $500 max - and you still wouldn't have that taken off your credit card until you went to approve it going through, so you could stop it before it happened.
I love this system!
Hope this helps
Miranda