Graphical improvements seems to be a good reason for most.
And per that same article, they noticed distinct improvements with the feature on. Sure it reduces framerates. Very rare that a feature that worsens image quality reduces framerates, right?
Depends on which graphical improvements you want though. If you have a monitor which runs at 2560x1600 you might want to use that res in order to remove the impact of scaling. Or maybe you want AA.
They are options which you have to pick instead of other things, rather than being able to use them as well as other features but at reduced (and still playable) frame rates.
If the cards could run it at 40fps average with all stuff on vs 50fps for ATI, then it wouldn't be so bad, but it can't manage playable frame rates with these "extra" options, meaning they end up being replacement graphical niceties instead of additional ones.
Either you have an ATI card and get stuck with one set of options, or you get an NV card and can choose either the same set as an ATI user, or you can use some different options instead, but not in addition to, those the ATI user gets. (And with either set of choices you pay more and get lower performance)
It ends up being something you need to experience to decide which option is most meaningful to you.
(But one game shouldn't persuade anyone to buy any specific thing unless that's basically all they play. But since the conclusions are specific to the game, the conclusion is valid as the authors preference is for the nicer water).