It's been said a million times, but no one is making you use any particular Pokemon. When people use common Pokemon, it's because they're good. They work. If you want to use a lower tier Pokemon to fulfill a certian niche, then who's stopping you? And contrary to what you seem to believe, some of the best teams in the metagame utilize at least one or two lower tier Pokemon. Heck, Lavos Spawn's new Sun team contains both Victini and Xatu, two UUs that work great in OU, and that Sun team has become a model for plenty of Sun teams used now. Other good examples of lower tier Pokemon with OU niches include things like Victreebel, Slowbro, and Quagsire. Again, using these is great, because you're actually using intuition to be competitive. However, creativity for creativity's sake is just bad.