I agree its better for US grads to defer graduation or work the research year earlier into the schedule (eg between 3rd nd 4th year). However, for those hoping for certain competitive fields that want research (eg ortho), it's actually not all that rare to do a Research year right after med school, and so for that purpose it wouldn't be as much of a red flag. If the research is valuable, I know some programs certainly aren't giving applicants a hard time about applying during a year at the NIH etc.
But I would only suggest that this is something that could help if you are a US grad who has borderline stats for something competitive. It's not really going to help an FMG as much, although I have seen a few FMGs that have made good connections during such a year that helped them get a foot in the door at a program. For an FMG I agree doing well at a good prelim might often be more beneficial.