Well, apart from agreeing with trent05, it's also a question of personal preference. Do you really feel comfortable signing off independently after four years of combined AP/CP? If you have ability, and have received good training, you might be that lucky. But for a lot of people, a surg path fellowship really helps them get more confidence in their dx'ing skillz.
Apart from that, some fellowships will, of course, all other things being equal make you more attractive, and thus more expensive to hire. Both as a greenhorn and down the road. That includes the boarded subspecs and certain non-boarded, like GU. That's not to say that you can't lead a long and happy life without such training, but something which should be taken into consideration. Think of it this way:
Group A: AP/CP generalists only. Can get a lot of primary stuff, but also needs to send a lot off for consults.
Group B: Has a core practise equal to Group A, but can additionally look at consults in GU/GI/Heme/Gyn/Derm, because they have the necessary subspec skillz within their group.
Which one would have the higher, and probably more profitable, volume? And couldn't it be argued, that referring physicians might as well ship to Group B in the first place, as they're likely to look at it anyway, sooner or later?