Let’s back up and understand why BS/MD programs like to see that students have performed some type of research. Most students who are interested in medicine want to do some type of research. That means that if you do research you look like a typical student interested in medical school.
Moreover, research focuses on the scientific process which is also part of medicine. By learning more about the scientific process you become more capable of performing well in medical school, and there are different techniques you can use to learn faster and improve your mind, you could visit sites as http://www.subconsciousmindpowertechniques.com/how-to-control-your-mind for this knowledge.
So, if you have a summers worth of research you will show that you are interested in medicine and understand the scientific process. That will make a BS/MD program happy.
Does that mean you are wasting your time doing more research? Not if it is something you really enjoy. BS/MD programs, and selective colleges in general want to see students that have a passion. If that passion is research, great. There are some BS/MD programs where that passion will make you a more competitive candidate such as RPI/Albany.
However, if your passion is volunteering, that is great too. For selective colleges, the passion is more important than the topic you are passionate about. For BS/MD programs the passion will typically be either health care related volunteering or research, whichever is more interesting to you.