I'm also delighted that PM is not an editor, but the crop/straighten function (along with metadata and culling) are among critical functions that I use all the time.
Crop is by far the least sophisticated. Helping it would not take much, and nothing that would require slowing it down. I like the OP's request, as I frequently am looking to just straighten (which is what is being asked).
Other aspects I've prefer seeing:
A "Select cropped" option. I try to tag all photos, then crop all tagged. Sometimes I screw up and untag by mistake. Being able to double check any images cropped and make sure all are tagged would be really helpful. There's almost every other "select xxx" option, like rotated, why not cropped?
Finer grid to more easily find a vertical item in the right places.
I think when, on a photo, I select a new crop aspect from the saved values, the current crop should change RIGHT THEN, and not have me have to pretty precisely click a corner. I just don't get it -- doing it in prep for the next photo seems the theme, but who does that before switching to the next photo?
A key to do a crop preview - show (just) the crop, straightened, full screen (or whatever you have on the screen).
Except for the latter these, and the OP's request, really are not bloat or likely to slow anything down. But the crop function is pretty unsophisticated now, and yet very central to PM for many of us.
