If you consider something like GTA V (@ 1440p given that's the res we're discussing), then even a 290X which can run at 54 fps on average can momentarily drop to 31 fps in near-worst case scenario (source:
http://www.gamersnexus.net/game-bench/1905-gta-v-pc-fps-benchmark-graphics-cards)
It's those momentary drops that made a big difference in our testing with this monitor. And further to that point, how many rigs are pushing 100 fps in the same circumstances?
If you can absolutely guarantee that your minimum frame rate never drops below 40 fps, then fair enough, but it seemed that for the majority of our customers they would occasionally experience that drop and the knock-on effect was severe. And as I said, the effect over 90 Hz was far less pronounced.
The point is, as you say, to find the monitor that's right for you. I'm not going to say this is the right one for everybody, because it obviously isn't - but I do believe it will suit the majority in this arrangement.