When it comes to ECs, everyone has a different say...
But I think when it comes down to the bottom line, you have to answer one question: why? Ask any pre-med who does lots of volunteering, they'd tell you because they want to have exposure to the clinical work, to get to know what it is like to be involved in helping/treating people, and this inspires them to go into medicine. All good, all legitimate. But I wonder how many pre-meds who do undergrad research could answer the question why they want to do research in order to get in a med school. because everyone else is doing it? or because my advisors told me so? YES, all the top tier med schools are research oriented and the quality of their research is what's boosting the name of each school. But, I'd also like to argue that doing research is important in your personal growth. Research is the tool to address scientific questions/puzzles - it helps you cultivate your problem-solving ability; 90% (hell, 99%) of time the results always turns out crappy - so it tests your patience, perseverance, and your creativity (to see if you are able to come around and solve the problem w/ a different approach); in addition, doing research also helps sharpen many skills like communication, leadership, networking, etc. Last but not least, biomedical research is always a very important aspect in medicine. (is this why some schools look so much into research?? maybe) Now imagine you were one of the committee members at a prestigious med school that's proud of its biomed research. Sitting across you is an interviewee who hates research (clinical, basic science, or disease oriented) and just wants to practice medicine. What would you think of this applicant?
I was very lucky to find a good research lab back at UCLA, a good mentor who taught me science (and wrote an awesome recommendation letter), a good graduate student who sparked my passion in research. PM me if you want more info but I got to tell you, my knowledge about UCLA is kind of outdated (been 2 years since I graduated).
Oh, just to be as objective as possible, I'm all for research, I love research, and am MSTP (don't throw stones at me...)