It's a fairly common practice. UAL has many times in the past taken a plane out to get guys current.

Back in the late 90's, we were taking guys out in the 737 to get the type ride because we didn't have enough sims. The DC-8 for a while wasn't certified as a Phase 3 sim, so we had to do bounces to finish the type. I went to Yuma. 1990 when we were in contract talks, the 747-400 was a new type, and management was flying the plane for their currency because we didn't have pay rates...and it wasn't flying due to the contact.

I'd imagine that when the Max gets back in the air, there is a good chance we'll take out a plane or two to augment currency issues. Mather is SF bay area would be a good choice. Palmdale, Long Beach, Victorville, San Bernadino Co--old Norton AFB, just to name a few. Moses lake in Washington. Heck for that matter, I was just in EWR, and Cessna's were doing stop and go's. No jet traffic.

Anyway, yes the sim is cheaper, but there are many reasons to go fly the plane if the need arises.