My stance on this subject is this: NU Pokemon are not bad. I've created what is similar to a theory. Basically it's this:
OU Pokemon can stand on their own with little or no support from the rest of the team. The Pokemon in this tier are the best at their specific jobs, but like any Pokemon, they have their flaws. However, these flaws are also very minor, such as Gengar really only being weak to Pursuit and the rare Sucker Punch. These Pokemon are the ones to go to for low-risk, high-reward Pokemon.
UU Pokemon can be just as tough as OU, but they are either too unpopular or require some support from the team. I use many UU Pokemon, as Miltank is one of the most annoying Pokemon on my team, and Azumarill can sweep through entire teams with the right support. However, like I stated, Miltank isn't very popular among others, and Azumarill needs a bit of set-up to release its full potential.
NU Pokemon may seem like the bottom of the barrel, but with the right team set-up, they can be just as dangerous. With a bit of creativity, you may find your opponent quite flustered with your surprise Pokemon. The only thing about NU Pokemon is that they usually carry flaws, such as a bad weakness to common threats, but they can be avoided with the correct set-up.
And that is my opinion. I normally don't pay attention to tier rankings, but you must make sure that your lower-ranking Pokemon have good synergy with the rest of your team.
QFT. Belly Drum Linoone works wonders with sleep support. Just a one turn set-up, and it can sweep entire teams.
I can't really justify Regice being NU, though. Yes, it might have a 2x weakness to Stealth Rock, but so do many popular OUs. The RestTalk set, in particular, is amazing in OU, and doesn't require any support whatsoever. People say Blissey does the job better, but Regice still has some advantages over Blissey (not always getting owned by physical attacks, higher Special Attack).