Several years ago this was discussed on what must have been an old forum. It was possible to change code in Mypaint to get a square brush.
I love some of the newer brush engine settings in the development branch so I have renewed interest in this app. Some beautiful brushes are now possible with these new settings. The limitations before were that brushes tended to be soft and round. This has now been improved. A round brush dab is obviously the most useful but I loved the square brush. Some very nice geometric forms were possible which are not possible with only a round brush. This is especially the case given the extremes of possible settings for the brushes. No matter how squeezed the ratio of a round brush is it always retains a softness. Some programs do provide other basic brush forms like squares and various polygons without using bitmaps. A square brush is definitely useful although not so much use if you just want smooth flowing brush lines. I wish this was an option in the brush settings. Currently to get this effect I have to use Gimp or Inkscape. I have attached some rough quick tests I originally did with the square brush. I think I posted at least one of these on the old forum. Also, look for an image by Sparth called Night Scene. He did some images with highly geometric forms which are very cool but not really possible with round brush dabs.
Should Mypaint ever have a transform tool that can freely alter perspectives then a brush for creating grid like patterns becomes extremely useful. This is a common trick that artists use in other programs.