I admit, encountering a Pokemon who uses Double Team can get
very aggravating (especially when employed by Blissey or Crobat), but it is a valid tactic. The same goes for the other stats. If I don't want you maximizing your Attack, well, I'm going to have to devise a strategy that and combat and/or counter offensive boosting measures. Just the same, bring something that can stop Double Team. You have Haze, perfect-accuracy moves, and forced-Pokemon-switching moves at your disposal, not to mention a myriad of strategies you could put into use. A move's status as being "annoying" to the majority should not be grounds for forbidding said move.
One thing advantageous about banning Double Team is that it helps prevent "endless battles". I remember in Emerald's Battle Factory, I used a rental Vileplume against a opponent's Crobat. Both of them were holding Leftovers and knew the move Double Team. The battle dragged on, and boiled down to the point where both Pokemon could only use Struggle, had +6 Evasiveness, and had a Leftovers heal each turn. Frustrating as it was, I had to turn off the game and end the battle, which ruined my winning streak. Struggle's damage was merely negated by Leftovers heals repeatedly, and missing meant the opponent got to recover additional damage. I think I played that match for nearly an
hour, with no end in sight. Had the Double Team-Leftovers thing not been in place, I believe the battle would have ended
much quicker.
And I hate how items like Scope Lens are banned, yet
Absol with Super Luck is still permitted in competition. And Absol is entirely capable of having another non-luck-based ability, at that.
As for one-hit knockout (OHKO) moves...I feel banning those is very justified and understandable, though I am somewhat open to allowing those moves in a battle. If used, they essentially nullify the user's Attack/Special Attack stat(s), the opponent's Defense/Special Defense stat(s), and the opponent's HP stat. Plus, the move effectively has "infinite" power; 30-point accuracy is hardly going to deter its effectiveness. Three
entire stats removed from the equation. And at least the ones like Fissure and Horn Drill/Guillotine are slightly balanced; Sheer Cold (Ice-Type) doesn't even have an immunity to contest it! The point of Pokemon battles is essentially to whittle the foe Pokemon's HP down to zero. If the HP value is rendered meaningless, there's not really a point, as anyone can just press A and get an instant-KO. Theoretically, OHKO moves are the only moves that can make any battle last only
six turns. Perish Song isn't nearly as punishing, as it does not decide fainting on the turn it is used.