How far out of residency are you? It's easier to maintain your skills with 4 shifts/month if you've been doing it for 10-20 years. It becomes ingrained. If you're fresh out of residency, you haven't obtained the "hardwired" settings where procedures become second nature to you.
The bigger issue most groups face is reliable coverage, malpractice coverage, administrative overhead, etc. It becomes cost prohibitive to spend all the money on malpractice ($30-60k/year) if the doc doesn't work that much. The malpractice cost is per doc regardless whether you work 17 shifts/month or 2 shifts/month.