A lot of places allow RNs, PAs, NPs, and physicians to just complete the exam, practical, and ride alongs. It appears that NY requires everyone to complete an exam, but allows courses to exempt or allow students to test out of sections if they have prior education/training at or above the certification level. The only reasons I can think of off the top of my head for a physician to maintain a prehospital cert is to meet minimum staffing levels if they actively work on an ambulance, street cred, and pride.
Of course a sticky situation is that physicians aren't bound by any level's scope of practice. There is nothing stopping a physician from practicing advanced care even if they are only certified as a basic. Of course they will have to rely on their own malpractice insurance if they do practice under their own license. An interesting discussion on this can be found here: http://forums.studentdoctor.net/showthread.php?t=428001