We can't get enough pediatricians to fill many of the more legitimate subspecialty needs in many places. Pediatric pulmonology, rheumatology, nephrology, etc are all undermanned. I doubt there will be such a large cohort of people willing to do a hospitalist medicine fellowship that they can actually saturate the entire hospitalist market or even the academic hospitalist market for that matter. It might make sense if you want to be competitive for jobs at the more sought after academic centers, take leadership roles, or have a research heavy career. I would try to at least get an MPH or MBA out of it but ultimately I'd personally have a hard time doing a hospitalist fellowship when I could use that time to pursue a better paying or at least more in demand sub-specialty.
The irony is that, as with most of medicine, the career paths that will most require the extra training will be the type of career path that pays the least. The community physicians in less desirable locations will continue making twice as much money and shaking their head.