I think it would be mightily useful, when you want somewhat rounded stuff but can't pixel it out without ugly jaggies.
edited December 2016
curved line maker? i second this
I was thinking about something like bezier curves in vector programs, but then rendered to pixels. The kind of spline tool that was in old MSPaint (pre win7, maybe even win7 one) is bloody awful as many times you need to do few tries until you get it right.
With this, bezier-like curve tool you'd just draw it, manipulate using bezier handles and once you're happy with how it looks, rasterize it into current layer.