The common saying is that first impressions are made in the first few minutes, then you can start relaxing in a traditional interview. So really a trad interview has 5 min of high energy impression and 25 minutes of lower stress talking.
In an MMI, the high energy intro is all you have... Repeated 8-10 times. There really is no "lets relax and get to know each other better" phase. Keeping that level of energy up for so long is a little draining.
Also, when I was at one school, I was interviewing with a bunch of people that it was their first interview. They were all stresses out and the anxiety in the room waiting for the MMI was painful. I was even in a position where I had nothing to loose; I already had two acceptances to other schools and that interview that I was at was going to be my last one for the cycle. He other interviewers were stressing me out.
dsoz