I'm guessing, but the only reason I can think of for leaving rthe governor on is so that you can't forget to put it on before flight and end up with an overspeed. Well-intentioned perhaps...but definitely not a good idea. Robbo drivers, how often have you pulled full carb heat to check it during start-up, and had the RPM go over 80% because you've got carb ice? I have, loads of times. And as a student, it's something you don't think about, until it happens. And you're doing most of your checks at 75% RPM, which makes it really easy to just wind up the throttle that little but too much; and maybe you close the throttle to check the needles separate, aren't sure that they really did, so what do you do...wind on a bit more throttle and start from slightly higher RPM....
Maybe there's a good reason for this change, but I definitely can't see it. and as many of you know, I'm pretty sensitive to the words "governor", "overspeed", and "RRPM", for much the same resons as The Nr Fairy.
