Personally, I don't think it's exactly nice becuase horses were made to move and roam when needed, but I don't think it's necessarily cruel.
If the horse has water and hay (and even if they don't, I think a couple of hours without hay/water isn't going to seriously hurt anything) and there is a purpose for being left tied (teaching patience because that's a skill the horse is lacking, not leaving them tied becuase they were "bad" or something) I think it has its place.
I think it could be easily misused though so you probably want to be careful of that. I'd say that 3 hours is probably the longest a horse should have to stand tied. I mean, that seems like it's long enough to get the idea of patience across/get the horse comfortable with standing tied and I can't really imagine a reason to need to have a horse tied for longer than that... But that's just my opinion.
Fabio - 13 year old Arabian/Lipizzan gelding
Rest peacefully, Lacey.