I see mounting to a stud as something useful if the outlet is expected to get a lot of use. The “wings” of old work box might possibly dig into the drywall and become loose. In this case the use level will be low, right? Besides, attaching to a stud is work intensive, and I believe that using an old work box like this is up to code. I’ve used them a number of times.
EDIT: Also about the caps… I try to observe a “code plus” approach and wrap caps with friction tape (NOT the plastic electrician’s tape). That stuff will provide a little more protection for decades.