Fairly untrue. I'm sure we have all seen YouTube videos where it's time lapse of someone drawing something AMAZING in MS paint.
Probably the only thing restricting art working from being produced in MS Paint is the artists imagination and creativity restricting them.
Yes if you want to make signatures and what not like pretty mcuh all the other ones you see then grab Photoshop. I hope you don't get Photoshop for that reason though. All those signatures people do, they are awful. They look so similar, it's like everyone that makes them are reading the same tutorials.
Edit:
Here I went ahead and found some interesting videos for you.
Mona Lisa in MS Paint
http://www.youtube.com/watch?v=uk2sPl_Z7ZU
MS Paint Car
http://www.youtube.com/watch?v=vUWqRhReaZk
Drawing Nazgul in MS Paint
http://www.youtube.com/watch?v=6iSPqXqcuWo
There are MANY, many more, but I won't bother pasting nine million different links. I'm sure you can find your own, I just wanted to give you some examples.