It depends on the games you play, if you're only playing cod for the campaign it may be a waste, but the MP playing 120 vs 60 is night and day. It adds to response times, when people can die in milliseconds, every millisecond matters, and at 120 you would see an enemy pop out faster than 60Hz would show, and the image stays smoother in motion so if you have to track and shoot at a moving target, you will be more accurate @120Hz. Basically, 120hz is for competitive players. on PC, we've been turning down the graphics to get more frames for years, play a day @60 and then 120 the next day, you just end up doing better with more frames.
Does the online separate 120fps mode from 60fps mode in match making?
Resolution and draw distance are also important for competitive multiplayer to distinguish the enemy from the background. Turning down the graphics helps on PC to remove 'clutter' and make the enemy stand out more. Turning down the resolution (what the consoles do to achieve 120) doesn't help.