I think the more competitive a field is, the more likely that research and research in the area is highly looked upon...
If you look at "charting the match" on the nrmp website
http://www.nrmp.org/data/chartingoutcomes2009v3.pdf
The specialty with the highest number of research experiences are: Rad Onc (3.8 matched, 3.6 unmatched), Plastics (3.7 v 3.1), Derm (3.6 v 3.0), ENT (3.4 v 2.9), Neurosurg (3.3 v 2.8), and Transitional year (?, 3.1 v 2.7). a few specialties had fewer in the matched vs unmatched (FM 1.3 vs 1.8, Peds 1.8 v 1.9, Med/Peds 1.7 v 2.2, OB/Gyn 2.1 v 2.2, PM&R 1.8 v 2.1, Psych 1.8 v 1.8)
There is probably a lot more that goes into it, and you can also look into each specialty one by one... In rad onc, 7 people reported no research projects, 5 matched, 2 did not, 11 did only 1, with a rate of 10:1. 5 or more, rate of 41:7 (so, statistically speaking, it would have been better odds if you only did 1 project vs 5 or more). I think the value of research is greatly overplayed, and not having research will not make it impossible to get a spot, just like having research will not assure you a spot. If you have a field you really want to go into, do whatever you want to do to make your application strongest you can. Clerkship grades (both 3rd and 4th year), Step scores (both 1 and 2 if you take it) and Letters of Recommendation all have more impact than a research project does, so don't forsake any of them just to be able to put a research project into ERAS.