A decent MD candidate will usually be a decent MD/MS candidate. It is just not that selective. The main bottleneck is that you need to prove you know enough about research to commit to doing a year or two of lab work full-time.
I personally think an MS year is a great experience for just about anybody in medicine and this pathway has been sorely underutilized, so good for you for considering it. Having done some research in a systematic way makes you a much better consumer of scientific data in your future career. And the MS gives you some credentials for your own future research if you should want to go that way.
