I wrote separate ones (well, I reworked my "main" PS from my specialty application) for TYs and prelims. I think it is essential esp. if you are applying to competitive TY/prelims. How reading about your passion for radiology is going to make admissions people make the decision on whether you are going to be a good intern (and not try to "game the intern year", as has been amply discussed in this forum). Don't be lazy (or at least, don't appear lazy to the admissions folks). You shouldn't spend more thab 30 mins re-tweaking your radiology PS.