From what my interviewer told me when I interviewed, there is a selection committee and they go over all your stats, ECs, recommendation from interviewer etc. If there are any huge weaknesses that suggest you might not have good study habits (low GPA, lower MCAT, or interviewer's thoughts) then they may suggest you for MERP. When my interviewer told me about it, I asked what I could do to avoid MERPing. At the time I was applying for September, and he said Sept was really almost full, but if I would consider May I'd have a much better chance. Low and behold I'm here on the island in May.

I will say about MERP that alot of the people I've talked to on the island who went through it found it to be a really positive experience (albeit expensive). The one advantage I've noticed so far is socially. Everyone from MERP already know each other etc. and it can be more difficult to break into those social circles as someone who didn't MERP.