The Combined programs are Independent programs -- they have to follow the same rules that traditional Independents have to follow. I counsel medical students to focus more on Integrated programs than Combined programs because of exactly what you said. Integrated programs need to focus more on their junior residents than Combined do -- if they have a washout, it can be a real problem. If a Combined resident washes out, they just fill in the SFMatch. No problem. Also, it's a gentleman's agreement -- they don't have to actually take the resident into the program. In addition, there is a question as to whether or not the required rotations would be applicable if done during preliminary years in a Combined program since they aren't technically part of a Plastic surgery training program at the time.
My answer? All programs should shift to a 2+4 model (that has been tentatively approved by the Board for piloting at a couple of different places). Plastic surgeons should teach plastic surgeons. Spend more time doing flaps, facial fractures, hand surgery, and aesthetic surgery.
But that's just my opinion.