Seeing as though this thread has come up, I'll add in my updated thoughts on this. I've recently bought a nose plug and wear it sometimes when I'm playing. This is due to doing some minor sinus damage to myself after lot of playing (and flipping/rolling) in quite a fast hole. Felt like crap for the next few weeks, figured it's not worth it.
In cold water when you keep rolling up and blowing the cold water out of your nose, u can put the nose plug on, roll up and just the small difference that not having to try and clear your nose out of cold water. Mentally I think it helps u feel that little warmer, or that you can stay in the cold water a while longer.
I still only wear it if I'm planning to be rolling constantly when I'm playing in a hole. Even at the beach I won't wear it. Although I'm only 'playing' I find I'm either paddling out or catching a wave. If you catch a wave and then have to roll, you still get a break as you head back out. Not like in a hole you can roll, catch the eddy and be back in there 20seconds later.