you can get a discount if you buy them through a gift card exchange like Plastic Jungle.
In response to the OPs question,
Anything negative you can thing of, this is always my answer/response.
I understand the money saving opportunities that you can get by pre-purchasing Disney GCs but there are two major reasons why I won't do it and the above comment is one of them. You can usually find Disney GCs for sale second hand through places like Plastic Jungle because people don't use them all. In the meantime, Disney already has your money. For me, it is not worth the savings I get to just let Disney have my money while I figure out exactly what I want in return.
Alot can happen before, during and after a WDW vacation and I don't want hundreds of dollars in Disneys account instead of mine if something does.
Another reason is security. Should I get pick-pocketed or lose my credit card and it is charged immediately, I can refute the charges and I have lost nothing while my dispute is being handled. I can still continue my vacation as scheduled and no money has been taken from my funds.
Now, if I lose a Disney GC, I can always call and cancel the card and have a new one reissued, but if the card was already used it can be a PIA.
My daughter got a $50 Disney GC last year for her birthday and we lost it. I tookma pic of the card so I had the number in my phone and by the time we called to cancel it and request a new card the money was already spent. With a credit card, there is more "protection" in that sense.
Lastly, overspending. I read not too long ago that more than half of the people who purchase gift cards spend more than the balance. This can mean that some are buying the proper amount of gift cards (which is less than they will actually spend anyway) but it also suggests that people are more willing to overspend when the decision to purchase was made so long ago and you are already "invested" into the gift card.