Arguments have been made for and against over the years and in the end I guess it comes down to how an individual utilises the computer and personal choice.
One of the most common arguments for leaving a machine turned on is that the constant heating/cooling cycle will eventually cause solder joints to crack and subsequently, poor or zero connections. I doubt that would actually be the cause of too many breakdowns.
One of the main arguments for switching off is because leaving it on will consume a lot more electricity. Seriously????? Have these people never heard of "Power Plans"; sleep mode and hibernation?
Me, I leave my computers switched on all the time. The only times I ever switch them off is:
1) If I am going away for an extended period and
2) If there are thunderstorms about.
That said, I do spend a lot of time on the machines and so it is mainly for convenience.