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"But I can't think of anything in PS that is better done with the mouse."
Plenty of thing including but not limited to very precise Paths, which are almost impossible to do with a Pen, and cloning along an already existing straight edge, where the alignment between the copy and the paste has to be down to the pixel level to avoid a dog leg in the clone. And menu items are just easier with a mouse.
I paint left handed and mouse right handed, so it works out well, with the Wacom on the left and the mouse on the right. Very efficient, but you have to be sort of ambidextrous, or at least it helps.