"* Most wear & tear on disks, fans etc occurs when they are initially spun up; far less when they are constantly spinning "
But in most PCs they are not constantly spinning, even when the PC is on 24/7...
Very few PCs run the HD 100% of the time, especially if power save features are used. Also, many new PCs only run the fans when the temperature hits certain limits.
"* Rapid rise & fall of temperature does more harm to the insides of your machine than constant temperatures - this is especially true of connections between (eg) cards & the motherboard where different compositions of the metals mean they expand at different rates."
Hmmm... tentative... The amount of metal is so small I doubt the expansion makes much difference.
So, unless you really need to run a PC 24 hours a day, such as with a server, or something that needs to access data all the time, then I would say that if you are not going to use the PC for more than a few hours, shut it off.