Yeah, this is a well-known tactic. They aren't absurdly common or anything though. That's because one Pokemon is going to have to waste a moveslot for Sleep Talk. With that said, this strategy isn't for every Pokemon, especially frail ones. Mainly cuz frail Pokemon are generally very good sweepers. Your Azelf idea sounds nice, since it's exploding, so it's not like it's going to live very long anyway, but it's just that you kinda waste it. In a way, you made Hypnosis become an instant KO move. Btw, one of the most common sleepers just happen to be a ghost type (Gengar).
Also, the likelihood that Azelf will pick explosion is rather slim. If Sleep Talk and Explosion are the only moves, then you wasted a team member, since its ONLY role would be to absorb sleep and/or explode. If you give it other moves, then making it become a sleep absorber wouldn't be the wisest idea, because then you can't choose the move you want. This is the reason why frail pokemon aren't used as sweepers, because then it'll be easier to slaughter. Ex with Azelf: 1) Nasty Plotting when you don't want to 2) Using Grass Knot on Heatran (
), etc.
Now, I'm not against exploding Azelf or any Pokemon for that matter (I'm all for it, actually), but when you think of your team, there really isn't a reason why you'd give it Sleep Talk AND Explosion. It just leaves everything up to chance; Explosion as the only move? I don't think so. Moves alongside Explosion? No, because what if you explode when you don't want to? Or don't explode when you needed to?
Now, there ARE some good sleep absorbers, like Machamp, Milotic, Zapdos, and pretty much any Pokemon that uses either Sleep Talk and/or Rest. This is because 1. They're bulky 2. They use rest and sleep talk. 3. They have no better moves. 4. Has an ability that benefits from being asleep/statused. It could be a combination of any of these. A Pokemon with an activated Toxic or Flame Orb could absorb status as well.
All-in-all, it's a good strategy, especially since sleep is one of the most crippling statuses.
The thing with Uproar is that it's in the same boat along with Explosion (minus the great damage potential); the chances of you waking up are rather slim if you choose to use more than 2 moves (sleep talk and uproar), but if you use 2 moves, it's just stupid.
