The time you spend reaching for the paintbrush like Pawige says, could be spent on working on the piece. The less the mouse pointer leaves the art view, the better you're using your time.
I'm all for restraints, but there's artistically charging restraints (like working around 2 colors for 8x8 in c64 hires mode) and there's artistically retardant restraints (like for example, pixel only with your pinky on the mouse). MSpaint comes closer to the second variety than the first. Almost everyone that comes to the defense of MSpaint should better try Pro Motion (the demo for example) for a couple of weeks before passing judgement. I can pass judgement because I used MSpaint for years, years ago. If I hadn't any MSpaint experience, my opinion wouldn't count for much.
yeah, i was talking more the physical restraits, like having to change tools or click on the palette every time you wanted a new color, which is not extreme as matthew barney (google him if you dont know the kind of trash he does), but MS paint is still a hinderance. I like restrictions like oldschool specs or even things like only use pencil or only red, black, white, and ochre, or only sepia ink. Of course, mixed-media is fun as well

i cant officially digg it because im too lazy to get an account