There's cons to both ends of it...
A: Leaving them on 24/7 surely shortens the lifespan of components throughout the entire machine. The longer it's on, the higher the temperature will be inside the cab and right on the boards where the heat is generated.
Then there's your hydro bill...and the fire risk.
B: Just like a car, constant startups are NOT good. In the case of our machines, the sudden startup is a good jolt. If you measure startup current, it peaks higher and then settles.
I think, if you're turning your games off and on several times a day...just turn em on and leave them on for the day. Shut them off before bed.