I much prefer arc-seconds for things like HFD - because it tells you a size that is independent of the optics and camera and binning. If I know my focus curve goes down to 1.5" that is much better than mapping from 2.4 pixels and whatever binning.
It also can be used to reject stars that are much smaller than they could possibly be - which is the main problem I have with autofocus and star detection. So I will enter a separate request for that.
It does require having the pixel size and focal length correct - but people should really enter that properly anyway.