I think of the tier system as it is as more of a guide to what's most useful and what you're most likely going to have to have answers to than a list of pokes that you absolutely need to use. The OU tier isn't a list of the ONLY pokemon that can compete at that level, it's a list of the ones that you're most likely to encounter because they're generally really good, good enough that they're banned in NU, UU, and BL, but not necessarily good enough to compete in Ubers. But if someone finds a pokemon in UU, like Tentacruel, Froslass, etc. that performs at an unexpectedly high level in OU, then it totally makes sense for it to move up to OU, so long as enough people use it, in which case it needs to be added to the OU tier listing so that people will know that they need to be prepared for it. Feel me?