There is no doubt these machines can be left on indefinitely but that doesnt mean they *should* be, or need to be, which really depends on the situation. They are designed for continuous use which is different to being designed to be left on indefinitely.
That said, in a working environment where you expect to walk up to it and use it at a moments notice any time of the day, leave it on til you leave for the night then switch off.
In a home environment where the machine is definitely not going to be used while you are work, only being used once in the morning and once at night, and it is home alone, switch it off after use if for nothing else, for the sake of safety.
I do exactly that. My Diadema Junior EXTRA Pidd'd Dual Boiler cupping machine at work is in all day until we leave, and my Unico Splendor at home is always off except for when we intend to use it.
All you need to note, is a machine that is on for long periods, whether in use to make coffee or not, is still cycling and will suffer the "usual parts" failures sooner than a machine that is not left on all the time. By "usual parts", the ones that come to mind easiest would be pressurestats and seals. Pressurestat switches burn out, and seals are affected by heat, go hard and leak etc.
Hope that helps.