I think research is very important for research oriented schools (i.e. USNews) All things equal, sustained research experience probably gives you an edge in applying to heavily NIH funded programs.
On the other hand, research is absloutely not necessary for regular MD admissions. A lot of people just hated doing research, and I understand why. It's very much a delayed reward.
Keep in mind though that for regular MD programs your clinical experience is probably much more important than your research experience.