My desktop is, and always has been, on 24x7 with no mechanical HD sleep.

On the other hand, in order to save power, my family's desktops all go to sleep (not hibernate, which isn't needed on a desktop unless you're really worried about unsaved data and losing power).

Yes, parts have gone - usually power supplies - and those almost always go on a power off/on cycle, which DOES stress everything the most. But it's happened to both my desktop that's on 24x7 (usually fails when I turn off to do an upgrade or cleaning or something), and to my desktops that go to sleep all the time.

Believe it or not, I've had power supplies fail on desktops that were NEVER turned on, but just sat supplying standby voltage for a couple years in a row. In fact, that's happened twice on the same system (if you care, an old DOS Pentium system I keep around for nostalgia purposes).

Now, as for mechanical hard drive start/stop cycles - yes, those will kill drives. Manufacturers rate drives on the number of cycles before failure. But I've seen proof that drives are a lot more resilient than we think.

Here in Toronto, a certain generation of Rogers DVRs were poorly programed to have sleep mode on the drives enabled for like a 10 minute timeout, but then to also do cleanup duties in the background. So, when the boxes were "off", the drive would soon shut down, and then a few minutes later start up again to do some minor task. Then 10 minutes later, shut down. Then start up again. Over and over. For YEARS! I had 3 boxes doing that (including one in a bedroom that drove me NUTS), and yet I never had a drive fail.