Well, like the folks said, the biggest advantage is mobility. I still prefer to hunt in hang ons and permanent when I can, but it's a necessity to own at least one, in my opinion. As for noise, I can get up in my summit cobra a lot quieter than anybody can screw in a bunch of steps and hang a loc-on, guaranteed. You won't hear me climb from 40yds away, and on most trees, from 25yds. The newer ones are real quiet.
Also, safety is a big factor. It's easy to wear a harness the whole way up, and never be seperated from your safety line. No slippery steps either. No stolen stands on public land, and nobody can hunt your spot without bringing their own stand.
However, like I said, I prefer fixed stands, because it's 20 less pounds I have to carry in. You don't sweat as much just climbing a ladder, so you can wear heavy clothes on the way in. I own a climber, a few hang-ons, a ladder stand, and a pop up ground blind, as well as a half dozen permanent stands. Every year I find uses for all of them.