I vote yes purely on principle. Pokemon is hardly a balanced game in and of itself. As a competitive community, we have taken it upon ourselves to build up a set of rules that minimize the game's inherent imbalances and try and turn it into a fair competition, minimizing the factors that take away from pure skill. OHKO moves are one of those factors. They are an element of random chance that can determine the outcome of a battle at the mercy of the RNG, and this is something to be avoided.
Playing with OHKO moves is not the same as the choice of Hydro Pump or Surf, despite how it appears on the surface. Hydro Pump is a gamble to use over Surf, thanks to shaky accuracy. To choose Hydro Pump over Surf means that the user is taking the risk of no payoff for the chance of a larger one. Arguments in favor of OHKO moves claim that it is the same with these moves, just on a larger scale. However, the difference between OHKOs and other high power moves is that even the most powerful moves can be played around. If I predict Ice Beam or Blizzard, I can switch in Vaporeon or Milotic, and I have checked the attack. However, you can't play around OHKOs. You cannot check them with anything but very specific counters, especially Sheer Cold, where there is no type immunity. Unless you switch in these specific counters, you are at risk of losing something no matter what you did. Where is the skill here? If I switch in that Milotic expecting Ice Beam to get reamed by Sheer Cold, did I do something wrong? What could I have possibly done to prevent this? Anything that removes skill from the hands of a player such as this does not belong in a competitive environment.
I believe that we need to firmly realize that we are not a pure Pokemon simulator. We are attempting to play the game at it's most competitive, despite the random elements involved. While we cannot control the mechanics of the game, we can alter what elements we allow into it. Whether they would see widespread use or not, anything that directly removes the influence of skill from the hands of the players does not belong in the environment we are attempting to maintain. So, despite their impracticality, OHKO moves should stay banned for the sake of our principles.