With Sand Veil+Brightpowder, you have a 72% chance of missing when hitting Garchomp with Ice Beam on the first Sub. After two subs, you have a 51% chance of his not having a Sub. After 3 Subs that becomes a 37% chance. If somehow you still manage to break that Sub, there is only a 26% chance of breaking the next one. Really, is that what you want to be facing?
People have got to stop using the gambler's fallacy when using probability to make points. The chance of breaking the sub remains the same at every point, it does not decrease.
Having said that, for most people I think it's more a matter of "I don't like hax when it happens to me". Let's be honest here.
It's true that for teams that abuse hax with brightpowder, quick claw etc...it will pay off for them once in maybe 5 or 10 battles. So we can comfort ourselves knowing that those who abuse this will actually be disadvantaging themselves by having no items for the other battles where hax does not help them.
Then again, we really can't...because we may be that 1 in 10 that hax abuse has cheated out of the win. We don't care that in the other 9 battles, they have been royally screwed by having basically no item, no hax activation and lost miserably. It only matters that they haxed against us in this one battle and we were screwed by it and cheated out of
our deserving win. And because of that, we hate hax and it must be banned.
I think evilbob has hit the mark here. It's the possible devastation of missing the garchomp and having it SD up that makes brightpowder so annoying. Because frankly does anyone really care if you have brightpowder or quick claw shuckle. Like "WTF! your shuckle outsped my ninjask! HAX! I'm leaving".
As people have said, these items have their pros and cons. It's a gamble...sometimes it pays off and sometimes it doesn't. Using an earlier example, sometimes thunder will hit and KO your pokemon when an expected thunderbolt wouldn't have. Other times, it will miss and I, in turn, will be KOed. It really is no different.
Although, and I'm the first to admit this...here, outside of the battle, I'm all for saying luck is part of the game and these items have their pros and cons. But in battle, when hax happens to me, I hate it and it shouldn't be allowed. :D