Quite a few bicycle builders are using powercoating, either exclusively or as an optional finish. My '97 Marble Peak has a solid powercoat finish, my '98 Ventana ECdM had a wonderful, translucent red powercoat finish and my '02 ECdM has what's called a "SuperDust" powdercoat finish which is pretty much a metal flake that looks stunning. Like wet paint, the skill and experience of the powdercoater is what willl limit the use of powercoating, within limits. Attention to prep work needs to be better unless you want to chase all of the threads and reface the bottom bracket and head tubes.
The benefits are that it is generally more efficient and economical to use compared to wet paint, there are some environmental advantages since there are no paint fumes, it doesn't take a lot of skill to apply so long as it's used as a base coat or solid color, and its very durable. My guess on weight is that when properly applied, its no different than a base / top coat paint job. If clears are added on top to bury or protect decals it then becomes the same process as putting clear on a wet paint top coat. Wet paint can actually get pretty heavy, e.g., my two Erickson signature bikes (one solo & one tandem) have seven layers of wet paint: base + color coat + 5 hand-sanded clear coats over the decals. The S&S bike has a standard 3 layer finish: base + color coat + clear.
You'll usually find that places offering both the option of powdercoat or wet paint will charge less for powdercoating given the aforementioned efficiencies. Builders who stick with wet paint most likely do so because they've already made a capital investment in paint booths or have a relationship with a good 3rd party paint shop that meets their needs and the expectation of their customers. Again, wet paint in the hands of a skilled painter affords a builder limitless options on finish details that are often times selling points for their frames.
Again, if you do some checking around I think you'll discover that there are a lot of off-road bike builders using powdercoating, as well as a few custom builders and bicycle paint houses, e.g., Cyclart.