+1 to pros not using clinchers or tubeless "most of the time" but it does happen occasionally during a TT.
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FWIW, from an interview with Jonathon Vaughters, Director of Cannondale-Drapac, in advance of the 2017 Tour de France Stage 20 ITT:
Ridemedia: "With regard to the specifics of the time trial, you have disc brake bike options; what’s Rigo’s set-up going to be?"
Vaughters: “We’re using just the rim brake bike. We’re using a front, tubeless aerodynamic wheel which is new technology, and a rear disc with a clincher tyres which is almost the same design as the tubeless but not quite.”
Vaughters Q&A: "There?s a chance..." - Ride Media
I also seem to recall Vaughters saying something on the telecast about the team, or at least some of their riders, using tubeless during the rainy prologue. Something about being able to use lower air pressure for added traction. I can't find a reference for this though.