Well, this would certainly fit my workflow. Because I'm not a 3D-programmer and this approach is a much better fit for the casual 2D-games I want to make.
I guess I can come around to how this might be useful for some effects. I don't necessarily think it would be useful for 2D games though, as there's a lot going on there so you might be better off to just do a 2D-canvas based game.
Just off the top of my head: cheap glow effect, ripple effect (e.g. when a bomb goes off), post-processing filters for things like film effects, etc. You don't need to be doing 3d work to make great use of shaders.