Which is why even testing it is a bad idea, unless the aspect of the pokemon is so overpoweringly the reason why the pokemon is broken in the first place. Dark Void is not that with Darkrai. But even testing something like this sets a precedent that could be used to justify pokemon-with-move bans that aren't anywhere near as reasonable as Darkrai's Dark Void (which isn't that reasonable itself). It will also take a while to test and be hard to quantify; because of this, even testing it is a waste of time and will probably cause more harm than good.
All we need to do is state categorically that we are only outright banning moves, not combinations of moves and pokemon, to avoid any slippery slopes.
The problem with banning stuff like Dark Void and not just sleep as a whole, is that you have to then do the same for other stuff which breaks Pokemon. Should we test a Shaymin-S without Seed Flare (which, like Darkrai is only potentially to be used on two or three Pokemon - Shaymin, Shaymin-S and Smeargle)? Or more broadly like someone said, a Salamence without Outrage; Latios without Draco; etc. Where do you draw the line?
We cannot ban Seed Flare because while Skymin is probably broken with it and Smeargle never uses it, normal Shaymin makes good use of it without being broken. Because there is a single user of the move who is not broken and does actually use it, the move cannot be banned. Similarly, Outrage isn't broken on Dragonite, nor is DM, so neither can be banned outright. The line is drawn at only banning moves outright, not combinations of moves on pokemon. If there is even a single pokemon who learns and uses the move without being broken, then it cannot be banned.
There is the flaw of your argument. The moves do not matter nearly as much as the users do. The fact that Spore, an obviously, numerically better move (and your Soul Dew comparison fails simply because it and the other items are not directly comparable; there is no item that can give +2 SAtk/+2 SDef) is not broken according to you, because the Pokemon they are on are not good enough. However for you Dark Void, the weaker move, is broken, because Darkrai has it. If you want Dark Void banned you need to explain why the move itself (and then obviously by extension, Spore) are broken, not just that it will make Darkrai manageable.
Also, Dark Void is not broken on all its users. Smeargle is not broken with Spore, so it can't be broken with Dark Void.
All moves are only as good as their users. A 100% accurate OHKO move would be near-useless on something with a stat spread of 1/0/0/0/0/0. Ember on Reshiram is better than Flamethrower on Smeargle
The Soul Dew comparison is slightly flawed as I have admitted, but the point is still relatively valid; most offensive pokemon, when given the choice between Calm Mind/Bulk Up and Nasty Plot/Swords Dance, will opt for the latter. It can be judged that a +2 in (Special) Attack is at least as good as a +1/+1 in (Special) Attack and (Special) Defense. Therefore, a +2 in both offensive stats is almost certainly better than a +1/+1 in offense/defense, meaning Light Ball is
at least as good, and almost certainly better than Soul Dew, and DeepSeaTooth is almost certainly on a par with it. And yet, Soul Dew was banned and these other items not even considered for banning. Why? Because Soul Dew had by far the best users. You can pretend all you want, you
know the reason that Soul Dew was banned was because Lati@s were far, far better than Marowak, Pikachu, Clamperl etc, not because Soul Dew was a better item.
Finally, Dark Void
is broken on all of its users. Users =/= Learners. Smeargle does indeed learn the move, but he never
uses it, since he also learns Spore.
EDIT:
Christ it's hard to keep up.
Wait, you mean "it doesn't make all (or indeed any) of its users broken", right?
Erm yeah, pretty much. What are you getting at?
Anyway, if we tested Darkrai without Dark Void, we'd have to try Shaymin without Seed Flare as well. Perhaps try banning both Judgement and Elemental Plates as well? Manaphy without Tail Glow should be given a shot, because Volbeat isn't all that viable anyway(jk guys, don't kill me). Don't forget Fire Dance-less Urgamoth. For that matter, Sand Throw isn't that viable on anything besides Dory (after all, who'd use Sandslash when we still have Dory?), so we could get rid of that instead.
Of course, it's not banning a certain move on a certain poke. It'd just be a blanket Seed Flare/Judgement/Sand Throw etc. ban, on ALL pokemon with access to those, so it's not like we're specifically targeting certain pokemon.
The problem with that is that we can't ban Seed Flare because Shaymin-L maks excellent use of it without being broken. Skymin is broken with it and Smeargle doesn't use it, but there is still
one pokemon that uses it without being broken, so it cannot be banned.
Judgement could conceivably be banned, but Arceus would almost certainly still be broken since it still gets all the elemental beams and lots of different coverage moves. The elemental Plates could also be banned since there's still Mystic Water et al (though that's something of a grey area), but again, Arceus would probably still be broken just because of its phenominal stats and STAB ExtremeSpeed (which before you ask could
not be banned as it's used to success by a multitude of other pokemon without being broken)
Tail Glow has Volbeat who's an excellent BPer.
Fire Dance
could be banned if necessary (assuming Ulgamoth's prevo doesn't learn in - does it?), but unless FD Ulgamoth is found to be broken, it wouldn't be necessary.
Sand Throw couldn't be banned because Sandslash uses it to success without being broken. It doesn't matter if it's outclassed by Doryuuzu (which btw it isn't, because it's less frail and priority-weak), if it uses the ability and isn't broken, the ability stays.
I'm aware that these don't actually reflect your views and you were just using them to counter my argument, but I just thought I'd show you how I'd counter each claim if they
did come up for real.
Spore is a better move than Dark Void, and Domeface himself has said that Spore is not broken. He said that because the users don't make it broken. If the users of a move make a non-broken move broken, shouldn't it be the user of the move that gets banned?
And as others such as TLK have said, where do you draw the line? There are certainly other ubers that could be used fairly in OU if a move or item or two were banned; moves and items that aren't otherwise considered broken. Are you suggesting that all of those get tested as well?
Define a broken move. You could argue that Flamethrower is absolutely superior to Ember, but what if Ember was exclusive to Reshiram and Flamethrower to Smeargle? Then Ember becomes better. The power (I don't mean BP, I mean like, viability) of a move is entirely governed by the pokemon who use it.
I'm not suggesting that the moves/abilities/items you refer to are all tested, because the vast majority of them will be used by other pokemon without being broken. In the case of other signature moves you could test it if you wanted to, but in most cases there would be little point - for example, even if we banned Judgement, Arceus has the likes of Ice Beam and Flamethrower to replace it for most types. It may make Arceus a little weaker (for example, he no longer has a 100BP Special Steel-type move for CMing with) but it's very unlikely to make him OU, whereas Darkrai runs DV on almost every set and there is no move he can use to reliably replace it on any of them (there's Hypnosis, but the accuracy is too appalling for it to be of use). However, if there was a signature move or a species-specific item that was broken on
all of its
users (no learners; Smeargle doesn't automatically qualify).