We just had a meeting about boards at my school and they said about 50% of students take both. And it's probably the people who are thinking of specializing, so for the DO friendly stuff, it's probs ok to send in COMLEX scores to ACGME residencies in primary care, ER, etc. I haven't gone head first into board prep yet, but I'm planning on just COMLEX and doing internal med/family med, but might switch my mindset as it gets closer.
Oh and they said something like 15% of the USMLE students did more than just Step 1. So it was just 50% take USMLE step 1, and of those 85 students, maybe 12 take anything more than Step 1.