I'm sure you already have your answer, but to clarify the previous answers provided, most 121 regionals do not seek an authorization in their ops specs to conduct circling approach manuevers in IMC anymore, and thus they don't typically train for it. (it is one less thing to do in that costly sim).

Additionally, in 2013 the FAA modified TERPS criteria, expanding the visibility required to shoot a circle-to-land approach in IMC. For a Cat C/D aircraft, the new visibilities required are often 3 SM or greater, so it's essentially a pointless manuever for a Cat C/D operator who primarily operates US domestic 121 flights.

XDashDriver I'm not sure what the approach speed of the Dash is, but if it was Cat A/B perhaps that helps to explain why Mesa actually authorized and trained for IMC circling in the Dash.

Cargo operators and operators who frequent countries like China where such procedures are more common are probably more likely to train for it. And the CAAC in China does require all circling limitations to be removed from the type and the ATP before validating your license/privileges.

Hope that provides more depth on the topic. Go read here if you're bored and geeking out on this Saturday night:

FAA Expands Size of Protected Airspace for Circling Approaches | NBAA - National Business Aviation Association