There is only one reason a user may wish to keep an unused Place and that is where, although being currently unused, it has previously had Notes, Media etc. added to it, so is a reference item.
I strive not to have any Unused Places, so Vyger inspired me to check my production RM database this morning. I had one Unused Place, namely Jeffersonville, Fayette County, Ohio. That's not a place that I research very often, if ever, so I was inspired to check into why I had entered it if it was now unused. I couldn't remember any research I had done that would have led me to Jeffersonville. And the fact that RM's place list doesn't really have a reverse order mode of operation made my quest slightly harder than it otherwise have been. Which is to say, if RM's Place List would have allowed to look at the Place List in reverse order, the solution would have jumped out at me. I would have looked first at Ohio, then at Fayette County within Ohio, and then at Jeffersonville within Fayette County, and the solution would have jumped out at me. Because within Jeffersonville as a fourth level I would have seen "his residence". Namely, there was a death at "his residence, Jeffersonville, Fayette County, Ohio". I must have entered it first as "Jeffersonville, Fayette County, Ohio" and then changed it later to "his residence, Jeffersonville, Fayette County, Ohio", thereby creating the Unused Place.
I don't think I ever would have found the answer to my question of where Jeffersonville, Fayette County, Ohio came from in my database just by looking at the Place List. I might have found the answer from the Place List by doing a Print of all the places with the Reverse Place Names option enabled. What I actually did to find the answer was using Find Everywhere searching for Fayette AND Ohio. The answer more or less just jumped out at me after running Find Everywhere.
Which brings me to a wish for an overall revamping of Place Management in RM. I was recently playing with the free genealogy software called Gramps - not as an alternative to RM but as a way to import RM's GEDCOM and immediately export it back out again to cleanse the GEDCOM of all the RM-ism's I could think of. Gramps is much too limited to replace RM as my primary genealogy software, but since I had it installed I played around with several of Gramps's features. I noticed that it manages places left to right, and the hierarchy of places is very front and center. There are no limitations on how many levels there are, so that what would be Place Details in RM is just another level in Gramps. WIth Gramps you can clearly identify every level as a country or state or county (or kingdom or a duchy or a fiefdom or a province or a district or whatever), and you can provide dates for any item. RM supports dates for places, but only in Gazetteer or County Check, not in the user's own data. There may be major glitches in the Gramps approach to Place Management that I didn't see in my brief investigation, but it seems clear to me the Gramps approach to Place Management is much more in tune with my thinking than is the RM approach or the FamilySearch approach or the ancestry.com approach, etc. I like the Gramps approach.
And to close out with Unused Places, I don't know if or how finding and reporting Unused Places works in Gramps. But in my case it probably would have been a non-issue. I would have started out with a state of Ohio containing a county of Fayette containing town of Jeffersonville. Then later, for the death event in question I would have added another level of "his residence". Nothing would ever have been Unused.