Dark Void's accuracy is absolutely irrelevant to the Spore vs. Dark Void discussion. 80% accuracy does not add more variance to the metagame. Dark Void itself adds this variance. If Dark Void had 100% accuracy than sure, we would be using Dark Void all the time, but that would have happened last gen too. Spore is one form of sleep that is stopped by certain Pokemon and abilities. Dark Void is another form of sleep that can be stopped by any Pokemon 20% of the time.
Also, if we're going with this argument, take Body Slam versus Thunder Wave on Jirachi and Togekiss in BW OU. Thunder Wave is the more consistent option with 100% accuracy, but is stopped by Ground-types. Body Slam is an alternative that can hit Ground-types, although it is stopped by the rather rare Ghost-types and only has a 60% activation chance (with Serene Grace). Isn't this pretty much the same deal? Yes, there are differences, and the difference between BH sleep and Jirachi/Kiss is somewhat pronounced, but isn't this basically what it boils down to?
For what it's worth, I would not be opposed to Sleep Clause in BH, nor would I be opposed to a lack of Sleep Clause.
I also don't use Spore on my better teams because I need all of my moveslots instead of using a sleep move and potentially limiting my coverage and/or having it backfire horribly.
#2good4sleep
I would be okay with Dark Void if it had 100% accuracy. Spore was not broken last gen, and slightly reducing the PP of Gen 5 Spore is a perfectly good nerf. What I don't like is the subgame of "Should I prepare for Spore, or for sleep in general?" and "Should I use Dark Void or Spore?" Here's why:
(Arcticblast: removed line breaks for easier viewing)
http://notepad.cc/share/wOzvxtH6mw
This is a table of your expected payoffs based on comparing your anti sleep provisions to your opponent's sleep moves. In constructing the chart, the following assumptions were made:
1. Having a grass type takes up less "team space" than using an ability to deal with sleep. That is why there is a worse payoff for answering spore with a dedicated anti sleep mon than there is with a grass type. Yes, I know some teams would rather have a Poison Heal mon than a grass type. That's fine, this chart isn't for those teams. This chart is for the teams that would rather run a grass type, since those are the teams that would be affected by the guessing subgame that Dark Void introduces.
2. There are no "special cases". This chart is not meant to cover the situations where a sleep mon loses to a mon that cannot switch into sleep repeatedly, such as a Prankster with Substitute and Baton Pass.
Payoffs:
The variable s is the value of the team space lost by "over answering" your opponent's sleep. Since it is assumed that grass types take up less space than anti sleep abilities, using an anti sleep mon when your opponent has no sleep moves is over answering them by two levels rather than 1. Hence, the value of -2s.
-1 is the payoff you get if your opponent can spore you with impunity. -.8 is the payoff you get if your opponent can dark void you, since dark void has a .8 chance of working.
Lastly, it is very important to note that -s is much smaller than -.8, the highest payoff you can get when you under answer your opponent's sleep. -.8 is enough to lose you the game unless you are lucky or have a serious advantage elsewhere. -s puts you at a minor disadvantage.
The Problem:
Running an anti sleep mon gives you the highest average payoff: -s. This means that it is a better option than the other 2. However, it is not the best option against spore. This means that it is often correct to run spore, which means people might start running... grass types. It introduces a complex game of RPS to BH without actually adding much or any strategic depth. Furthermore, this is a guessing game unlike many of the others in Pokemon, because it takes place during teambuilding. You are guessing against everyone you will ever play against all at once, and you cannot win all those guesses.
I make different predictions versus good players than I do versus bad players. Forcing me to make a prediction for all matchups all at once robs me of that chance to demonstrate skill.
This removal of an opportunity for skill, combined with the fact that it will increase the overall usage of lower accuracy moves (thus increasing in-battle variance) makes me think that Dark Void both lowers the skill cap and makes battles less competitive. The meta is better off without it.