well that all depends on your electives and program requirements, myself for example, if I were to persue a career in IM I probably would do an osteopathic internship for my first year and in that first year I would have to see peds, ortho, ob/gyn, ER, surgery.
Some programs in IM that I have looked into are now requiring residents to do womens health, maybe no OB, but yes, gyn. One program in particular that I looked at states that the "core" of their program requires every resident to do one ER month every year!! and definitely requires ortho!!. Some other programs in IM also offer a tract in womens health.
I do believe that internist are starting to realize their descrepencies in some areas and are now trying to compensate for everything but peds.
As far as "falling back" goes, I know that in most states, after you finish your internship you are "legally" allowed to do pretty much anything you want, well in the legal sense you are at least, but good luck trying to find a hospital that will give you the privleges to do some things that you desire, or finding insurance companies that will reimburse you.
Now if I may get philosophical, FP is a specialty that trains on how to manage primary care of patients "throughout their life cycle", yeah, well all know that in realilty that patients swtich doctors, etc etc , but really at the core of FP is the philosophy of seeing patients "throughout the lifecycle". To do this does take a person with a special personality, the "boards" realized years ago that yes anybody can call themselves a GP but to really do medicine "throughout the lifecycle" it does require specialty training thus FP residencies began.
In my opinion, and it is just my opinion, not a proven fact, I believe that the reason why FP and GP get so confused is because after FP training, many FP's take on a role as a GP, ie, they don't do OB, they don't do many office procedures, etc etc. I know there are people to the contrary but for the most part, this is what I have seen in the FP community.