Well, it doesn't hurt to have that on your application, but I would say that it also depends on what else you have to show for yourself on your application (clinical experience, extracurriculars, etc.). If I am not mistaken, it really depends on the school your applying too. If it's a research school, then it would probably be a huge plus. Then again I've also heard of people getting into Dental schools and not having had any research experience.
Reserach can be an exciting and wonderful chance to use your brain. Research can be both a challenging and rewarding experience . IF you are trully interested in Science, then explore. I don't know how many people come through a lab and just want the head researcher to write them a letter of rec, just so that they can have the reserach on the CV and or application.
just my 2cents.
I would imagine schools in which a good percentage of the graduates find themselves in academic or public health roles. Harvard is definitely one of those schools. Research looks good, but to be honest, I've never set foot in a lab, unless it was for my pre-reqs. I got in at UT San Antonio, which is supposed to look a lot more favorably on people who have done research. They even asked me what I thought of research in my interview there. I told them that it is very valuable and necessary, but I felt that it was not something that would make ME happy. I would focus on grades and DATs first and foremost, because ultimately that's what will get you in.
When applying to schools you want to look at both objective factors and subject factors.
GPA, DAT scores, demographics... those are all objective factors, and all schools will place great emphasis on them in the selection of a student.
Research experience, clinical experience, personal qualities... etc, those are subjective factors, and hence you can't really put a finger on how much they will help you in the eyes of the ADCOM. Nevertheless, they can only help you.