I think most people are approaching the OHKO clause with a wrong idea in mind. It's not banned because it is perceived to be overpowered, but rather because of how it minimizes skill involved. While certainly, using it in a smart way (not just spamming it on everything, using it to eliminate certain threats) will maximize the potential of the moves, they're still extremely luck reliant.
Basically, every turn, there's a random chance to do nothing or drop one of the opponent's pokemon. While risk management on the user's part can ensure that the misses aren't catastrophic, there's little that the defensive player can do (other than using a sturdy pokemon) to prevent the enemy from gaining advantage with a lucky hit.
While one could argue that using a heavy offensive team could minimize the risk that the OHKO move hits due to having less opportunities to hit, a fast or bulky OHKO sweeper could still potentially sweep through any team without much trouble, provided the sturdy pokemon are out of the way. It's very unlikely, but it is this unlikeness which causes the moves to be uncompetitive - luck decides whether they're useful or not. In fact, if they had 100 accuracy, they'd be much more skill based - more over centralizing as well, but quite obviously not luck based.
I don't think OHKO moves are inherently overpowered, but rather, they're simply too luck based - statistically, they're virtually useless without skillful use, but practically, it's simply too random - a game could be lost simply because the enemy had great luck with the OHKO accuracy, and there's not much the defending player could've done against it.
Personally, I'm against OHKO moves being allowed, ever. If they were changed somehow to require certain criteria to be met for them to work at all (e.g. Lock On/Mind Reader being used, accuracy boosts, etc), and upon meeting those criteria they'd work 100% of the time, they could possibly be balanced, but as it is, I don't think they can be incorporated into a competitive environment - its not that they're overpowered or that they eliminate any need for skill - certainly, a skilled player would still achieve more victories than a less skilled one in the grand scheme of things - but individual battles could simply be won or lost due to roll of the dice, which in my personal opinion is simply unacceptable.