Over here (uk) there was/still is a problem with certian sattelite TV boxes, mainly pace brand but some others, where by they are left on 24/7, simply due to their nature of use,
the tv is turned on and off as needed, but people forget about the sat box, or if it's a recording version it's left on to work that etc.
this was ok, untill the day you did turn it off, or had a power faliure, when the machine was turned back on, it's go pop and die.
cause was some sort of thermal shock, basicaly the system was always at a constiant temp... which was pretty hot at the psu board, when the power was cut, the temp dropped causing some components to fail, usually the electrolytic capacitors of which make up 80% of a psu.
When power was re-applied it blew the stressed components,
i can see a similar thing happening in arcade machines left on 24/7.
as has been said, if it's a mame machine, then why the hell would you want to leave it on 24/7? it's cot a consumer grade computer in it, not designed for 24/7 running, your asking for trouble,
and as for a 'real' arcade cab, why do you want to shorten it's life even more?
plus i guess you dont care about the electricity bill,