The problem with relying on a Sleep Talker is that it's horrifically unreliable. I've had multiple matches in which my SDef Rest + Sleep Talk Kyogre lost 1 on 1 to Sub Darkrai, a Pokemon its supposed to be able to easily counter, because it couldn't handle Bad Dreams + Hazards + Unlucky STalk rolls (i.e. the ones that weren't Roar because of Substitute). I couldn't just switch out either - since Darkrai was usually behind a Sub, it could easily just Void whatever I brought in and then I would be screwed. It got worse if hazards were up - your spinner is just going to get decked by Dark Void if it tries to come in. Even then, the only Sleep Talker that likes taking on Darkrai is Kyogre - Giratina has no business staying in on Darkrai, and Scarfed Sleep Talkers have to cope with that fact their revenge killing abilities are left up to dice rolls once asleep for the rest of the game (unless you run, like, one move and Sleep Talk, but that is significantly taking away from a revenge killer's utility just to deal with a status ailment or even a single Pokemon. Only Scarf Terrakion/Zekrom have the moveslots to spare for STalk imo).
You shouldn't be forced to rely on luck to counter a Pokemon. Even stuff like EKiller and Kyogre, both of whom are massive threats, don't really require luck to beat. In contrast, Sleep Talk Kyogre only has a 33% chance to get Sub Darkrai off the field with Roar when sleeping, eating Dark Pulses and Bad Dreams damage the whole time. This makes crap like Sand Veil and even Evasion moves look like a total joke.
IMO, locoghoul hit the nail on the head. Sometimes, it's not about Darkrai itself so much as the sheer support sleep brings to the table, especially with hazards factored in. I cannot understate how good sleep is as a support option, even if all it accomplishes is making sure you don't have full control of your moves. What if your Scarf Terrakion fails to pick CC with Sleep Talk against a set-up EKiller? What if SDef Kyogre picks Rest as Darkrai sets up Nasty Plot or Mewtwo sets up CM? Sleeping Pokemon, regardless of whether or not they are a Sleep Talker, are EXTREMELY easy to take advantage of by the rest of the team even if Darkrai itself isn't going to sweep. The deck is just stacked too heavily in the favor of the sleeper team.
If I get reqs (I'm close), I will be 100% voting to retain Sleep Clause. The luck it adds to the game is unnecessary and just ridiculous. It's not just Sub Darkrai either. There is something seriously wrong if freakin' Butterfree is sweeping Uber teams.