Russ, I don't recall the term Rare Soul being used prior to the coining of the phrase "Northern Soul". Maybe it depends where you were geographically though.
In the late '60s when I began to frequent venues in the Northants area that played rarer and more unusual stuff I'm sure it was just refered to as "Soul", or maybe "Old Soul".
My cousin had frequented the same venues a couple of years previously. When she found out I was going regularly she said " Oh, you like R'n'B then?".
So, within a few years it had gone from R'n'B to (old) Soul to Northern Soul. Same music, different label.
The trouble with pigeon holing is the preconception and misconception that goes with it, and I won't go into the derogatory comments that come flying from people who, mistakenly believe that they are in a different arena. I don't believe that they are. It's one big soul music scene, with variations, and if some stopped being so insular and defensive of what they percieve to be their speciality, they might find that the other pigeon holes aren't really so different.
I hope this makes sense - I know what I mean.