The only reason I can think of for having the "Use Aperture Ring" option, and its not a very good reason, is to prevent newbies from putting a manual lens on their camera, with it set to something other than "A", and panicking when the e-dial won't control the aperture.
Of course, now we have one or two threads a month, from some newbie who put a manual lens on their camera, with the aperture set to something other than "A", and is panicking because the camera is flashing "F---" at them...
I sort of agree with blackcloudbrew. It prevents you from using a manual lens unless you have some clue as to what you are doing. For the clueless, it is sort of a failsafe mode. Rather than allowing the camera to take wildly mis-exposed pictures, resulting in complaints that this Pentax camera is a POS, it won't function at all, until you get a clue.
I told you it wasn't a very good reason ...