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I think it would prove quite useful.
The free programs Kropper and iResizer will do the trick just fine for your purposes for now. In the future, it would be welcome.
I use Preview to crop image right now.
But a Crop function is needed to make PaintBrush functional. along with crop function allowing keyboard movement of the selection rect would be useful.
Crop is coming. Stay tuned.
Lead Developer, Paintbrush