I fully agree if you're talking about building infill stations for a RI intrastate rail service - increasing frequencies to pass the minimum "usefulness" threshold is where that conversation starts. But if you're building stations for MBTA service, I think Olneyville and Cranston could be justifiable. Pawtucket-Central Falls and Olneyville have very similar fundamentals within a mile radius of their (existing/proposed) stations...at least per the charts on p.7-8
here, combined population and jobs within a mile radius of existing/proposed station locations is a little over 50,000 in both cases. (Olneyville skews more heavily toward population than PCF does.)
It's true that demand for Boston-oriented MBTA service drops off south of Providence, but where that cutoff point lies is hard to tell from south-of-PVD ridership numbers because those stations only receive limited service. I remember reading in a RIDOT study (which I can't seem to find online now) that curtailing MBTA service at Providence actually doesn't gain as much capacity as terminating at TF Green. The recommendation was to make TF Green the Providence Line's southern terminus once intrastate rail was able to take over Wickford Junction. In a world where TF Green and any stations north enjoy full service, Olneyville would probably post similar MBTA ridership numbers as PCF; Cranston wouldn't be quite as strong (only about 40% as many people/jobs within a mile radius) but I suspect it would still be respectable; and TF Green probably wouldn't be further behind since it's fundamentals aren't too far behind Cranston's.