How would doing a fellowship make you get less money
While I can't say for sure, I would guess it could be because fellowships like SCI, TBI, etc., are in general working with patients on Medicaid, (ie., low reimbursement for the hospital/clinic). So it's possible that by specializing, you're doing less general rehab/MSK where you might have more diversity in payor sources.
That's just a guess though... My impression was that many PM&R fellowships just won't increase your salary. However, I know peds is a fellowship that can get you a very high salary, as there's a shortage of peds PM&R physicians, especially if you're willing to relocate (true of lots of specialties I suppose...). And I also think the market may get saturated w/ spine/pain physicians--seems to me like everyone wants to go into it.
I think the main reason you should pursue a fellowship is because you need it to do what you want to do--not for the money. There are so many things in flux right now we can't predict what will and what wont pay well in the future, so the best thing to do what you enjoy. If that happens to make you lots of money, congrats. If not, well, most PM&R doctors make good money (~$180,000-$200,000 seems to be the figure I hear and read most often) but they also have great hours relative to other specialties.