Concerning over heat on a G2 check this poor guy's post today: Gen 3 (2010-2015) Toyota Prius Forums | PriusChat
Took mine off years ago and yes it makes a difference when it comes to New England weather as the aluminum is oxidizing.... Removed my sister's 2005 Toyota solar and WOW what a difference the valve cover and hardware look new!!! Pictures attached of mine and my sister car..
If you think it's difficult to access or see the reservoir you ought to try checking the coolant level on some motorcycles. Plastic panel(s) have to be removed. Just plain dumb engineering.
I can't quite tell, but do you think one of the three mounting points is broken on my cover? It's rattling at certain engine r/min, which has just started after some recent heavy engine work: (make sure to select 60 fps for the frame rate; 30 fps gets a funky effect going that makes the wobble look super-slow!)