Lots! Agreed with above percentage range. I'd say in my program over the past few years, about 50%. This was also about the percentage of people I encountered on the interview trail who took at least 1 year off as well.
I know the MD/PhD path is a long one. Still, getting more research experience can only help you not only gain admission into a strong program, but also figure out more about your research goals, your interests, your strengths, if this path is definitely for you, etc. I would recommend it if you are at all feeling like it would be helpful and if you are in the position (financial, etc) to do so. 1-2 years is often really helpful for applicants who don't have as much research experience or who aren't sure if MD/PhD is totally right for them.