I like doing it; It reminds me of my childhood when I played with mud.
I think most artists have their favorite size or shape of canvas.
In some reasons, I like working on square. I like the shape since I was in college. (It is a long long time ago.) I feel more freedom in the shape of square.
Another shape I have in mind is a circle. Except some woodblock prints, I have not painted on a round shaped canvas, because it is much more expensive than rectangle or square canvases. But I am sure my work will be perfectly matched with round canvas. I hope to try it someday.