I think the big benefit it adds is marketability. You are set up to be a medical director of an inpatient facility. If you were in academics, you might be able to split your time and do both throughout the year. Not sure how much it would add in terms of salary. You would have the satisfaction of knowing how to handle a bunch of medicine issues without having to make calls to your peers in IM. Having said all of this, realize that 90+% of all rehab directors aren't double boarded and obviously do ok. You would also learn a lot of the general medicine while you are working as an attending. i.e. in 10 years, will you say "wow, I'm glad I got that extra training?"
all this coming from someone contemplating the same issues...
good luck with your decision