It's interesting that whenever I see a post like this, the question is always: "what's the difference btwn IM and FP??, not "what's the difference between Peds and FP or OB/Gyn and FP?". I guess it's because the differences in the later examples are implicit. But the principle making the diffence btwn (choose one: peds, ob/gyn, IM) and FP is the same. An FP gets cursory training in all these fields, where as the other specialties get in-depth training. News flash: all these specialties are considered primary care specialties. The most 'primary' of these specialties arguably being FP...
An internist is trained to care for adult patients w/ multiple co-morbid conditions and critically ill pts in acute care settings. It's true that residents trained in IM can sub-specialize. Keep in mind that the majority of physician's caring for pt's in metropolitan hospitals are general internists. The traditional setting of hospital and outpt based practice is continually being pushed toward internists. This may not apply in rural settings, where FP's still have hospital priveledges, but it's becoming less-and-less common in metro areas for FP's to care for inpt's, leaving their practices almost entirely outpt driven...
The match this year may be very telling of the big picture in health care economics. This is an arguable point, but in the eyes of third party payers (i.e.: medicare/medicaid), NP's, PA's and FP's are on equal footing in most outpt encounters. This is because the traditional FP outpt E&M visit is of low-acuity (e.g.: otitis, strep pharyngitis) in healthy patient populations. These 3rd party payers feel care provided to these pts can be equally given by nurse practicioners at 75% a physician's fee. It's true, in several states, NP's can bill Medicare directly (at discounted physician rates), w/o physician oversight. Think the rest of the 3rd-party payers won't catch on that these primary encounters can be provided at 75% less cost...? (Traditionally, what gov't entitilements programs do, private insurers follow in short order). This places entry-level physician providers in a tenous position.
If for only job security reasons, it's important to have skills that gives you something beyond what can be provided by an allied-health-care provider (i.e.: NP and PA)*. The advanced, specialized training in Peds, OB/Gyn and IM gives you the options to care for acutely ill and complicated pts that is beyond the scope of an FP's training.
*Disclaimer: I realize that an FP's training is significantly more indepth than allied health care providers, but this is a philosophical argument, and in the eyes of Uncle Sam, they are equal in many respects...