It all comes down to this: Is Pain a distinct specialty or not?
My feeling is: you pick a sub-specialty and do it full-time. Do you want to be a Pain "specialist" or do pain "part-time". There's definitely a point to be made for diversification and breaking up the monotony of a very narrowly focused sub-specialty, I will admit, but If you give up a years salary to do a fellowship in what is essentially a different specialty, despite some overlap, likely you think the sub-specialty (Pain) is better for you in some way, and worth pursuing. Does a cardiologist work as a general internist 2 days a week to keep up his internal medicine skills? I've never seen it, though it certainly could be done. Would you have your heart cath done by a cardiologist who really saw himself primarily as an internist, but just dabbled in heart caths a day or two per week to pad his salary a little bit? The same could be said for Pain.
I guess it depends whether you see yourself (and want the community to see you) primarily as a physiatrist who happens to do some Pain, or a Pain specialist who used to be a physiatrist (or anesthesiologist/neurologist/whatever).
Also, if you're that worried about Pain being decimated as a specialty, why waste your time on doing a fellowship?
I won't discount the potential doom and gloom on the horizon for Pain, but the likelihood of Pain ever paying LESS than non-procedure oriented specialties is very slim to zero. If procedures pay zero, then we're all billing evaluation and management codes making what non-procedure oriented specialties make (PMR, neuro, internal medicine, etc). They cant pay us zero dollars per year ( though they'd like too, I'm sure). Now, compared to a higher paid procedure oriented specialty like anesthesia, it's a different story, because Gas pays much more than PM&R or neuro, or psych. For that reason any drop in reimbursement makes it tougher to justify doing a fellowship, to then take a pay cut. Also, it can be brutal for someone used to a certain income that has to take a big pay cut.
So if you're PM&R, I say do Pain full time. I don't think Pain will ever pay less than PM&R.
Just one man's free opinion.