Firstly this is a pve game and even when pvp gets brought it it's not going to be competitive, and even if blizz goes back on their word about that it'll be completely separate from pve.
Smoke screen is fantastic, but I find it to be expensive enough that I only use it by itself. I would probably say the same about shadow power because they are both the most expensive discipline skills.
Honestly ALL of the discipline skills are fantastic and become even more so with their runes.
Take vault for example. By itself it's great and isn't too costly. Rune it with tumble, use the passives vengeance+perfectionist with 45+ discipline and you are looking at being able to vault a bagillion times. This would be harder than smoke screen to use, but if you're good at positioning this would be alot of fun.
Look at shadow power. You can rune it to 30% life steal. That's alot. You can combo it with cheap caltrops and have heals and snares galore.
I am being completely honest when I say not one skill I have used on any class up to 15 has been bad if you can use it well and work a build around it.
Smoke Screen is basically the only good defensive skill the DH has. Vault, Caltrops, and Shadow Power are all pretty unreliable when you get to higher difficulties/acts, since there are a lot of charging enemies, and in Hell they 3-5 shot you (and I'm sure in Inferno they will 1-2 shot you). Even if you stacked Vit really hard, I'm not sure how you would reliably survive some boss packs using Caltops, Vault or Shadow Power. Shadow Power especially seems to really suck, even with the Gloom rune, since it's basically a ****ty version of Smoke Screen.
So while this is kind of OP (even without any +max Disc, this is letting me actually not die in Hell to the tougher boss packs), to be honest I don't think a hardcore DH is even possible without this. They probably should lower the max Disc you can get from items. However the root of the problem I feel is that the other defensive skills just suck for the DH. If they nerf this too hard, the class will be very hard to solo with.
The Demon Hunter has probably the most powerful active defensive skills in the game because the Demon Hunter has probably the worst passive defensive skills. I agree with Troxism; the DH would become very difficult to solo with, without Smoke Screen.