Mandatory, no. Optional : Yes. Like said above, you would need to cool the back components, not the backplate. If the backplate isolate the heat and doesn't gather the heat of the components beneath it, then the 38 / 40°C the guy is talking about are totally irrelevant.
Also, making it mandatory will increase more the price of the HC. And I think it would be up to each to say if they really need to spend more for something they wouldn't want in the first place.
Basically it's a matter of need vs desire. Do you need it : we can't answer for sure since there's no testing yet on that card for this idea. But it's easy to assume if the card work fine aircooled, then watercooled is just for confort or pushing the card farther than air can (though currently it seems the power limit is the issue, not the temperatures). But you would need to use LN2 to push it to the extremes anyway so there wouldn't be a real need for those types of waterblock.
Now, can you want it : sure, you can desire it if you want. And if there's enough people wanting that, there'll be a product. There's already RAM blocks and HDD blocks, which only a handful of people would need, but that doesn't mean people don't want them even if they don't need it.
That's my two cents. :P