Not true. When the system heats and cools the boards expand and contract also each time you turn it on there is a rush of power through the now cool parts and they heat rapidly. Tis much better to leave them on for extended periods than to turn them off for 15 minutes or an hour and right back on again.No, only the bearings in the fan will suffer. Nothing else. Like I said, electropnics brake, they don't wear out like bearings, for example.
btw the electricity used is not that much really. It does add up of course but the PS3 slim uses about 100 watts or so. Your electric company charges per KWH here it is abotu $0.07 per KWH the PS3 would use 1KWH every 10 hours of use which comes out to less than 20 cents a day so it will not break the bank.