1) Integration of the MD and the PhD off the bat will improve your ability to integrate the two in practice to ask better tailored research questions and do bed to bench and bench to bed translations.
2) Shorter combined pathway, same training by cutting out overlap.
3) MD/PhD increasingly becoming the default pathway for physician-scientists.
4) If all else fails, I twice in my interviews mentioned the cost of MD training. That is a big reason why the MSTP was established, so you shouldn't feel too squeamish bringing it up if they shoot down your other reasons.