The main argument against it is the reason it hasn't yet been adopted - the panels themselves are simply too unresponsive for 120Hz to be a real advantage. Yes, you will get more 'frame rate juice' - but all this will do is exaggerate the hangovers from the slower response time. Don't get me wrong I think a 120Hz would be attractive if priced correctly, but the difference is simply not night and day if you're talking about 60Hz vs. 120Hz on an IPS panel.
If you want a real night and day difference - wait for OLED displays. These were the ones I was referring to that were 'several orders of magnitude' more responsive than any current LCD. And that is of course only the very tip of the OLED iceberg.