I wonder how many power pilots do check the ASI ion the takeoff run Also why is it not a check in a glider?
They do, to avoid the kind of situation shown in these videos:
<http://www.gliding.co.uk/bgainfo/safety/winch-safety.htm>
In addition, typically before flying solo, a student glider pilot will have had to demonstrate that they can fly an entire flight with all their instruments covered up.