Are you suggesting multi GPU?
It does seem odd that they havnt converted any of these yet...sure you can get watercooling other ways...I think there is an all in one GPU kit out there (it requires a different cooling plate for every card), but they are all really expensive.
~65$ for a corsair unit and ~5$ extra to get a bracket for the GPU. Being able to easily switch to either or would make it very usefull and the price is pretty fair already. Top end air units cost about the same, but you cant stick them on a GPU.
This gets you a much cooler quieter setup in an easy 15 minutes...theres nothing else really out there for GPUs that could do the same. Depending on the card you may need heatseanks for other parts, but cheap aluminum heatsinks could be made for those and would work well (probably better than getting sandwiched under a hot flat plate like usual).
Ive got a card now that has cooling issues...the computer doesnt get used because it keeps crashing. Cant justify the cost of a new card or some stupid expensive aftermarket cooler for it, but one of these would be great. Use it untill the card is no longer worthy and then I can swap it to CPU duty...or just move it to the next card. A simple cheap bracket change would be the most required to get it to fit.