Primary reason for electromagnetic devices to fail is the power surge at start up and the resultant warming up to a satisfactory operating temperature. Akin to light bulbs failing on startup, but not when constantly burning.
Hmm, I've heard that one before, but I'm skeptical. And not sure the light bulg analogy is really suitable. Even shut down, if still connected to power, there's trickle current going to the hardware, if I'm not mistaken. Anyway, there are posts from people who've shut down at night for ten years straight who've had no problems and then from users like you who swear by the opposite. A lot of this is just too anecdotal. I still need to see convincing statistics; and I don't think there are any.
And even if you're right about the startup power surge and temperature jolt to the drive, you'd still have to weigh in the possible negative of too much heat, too much of a good thing by never allowing it to sleep. Must also depend on ambient room temp. It was just 103 here (90+ inside) and I don't think I would have been helping the hardware last by not allowing the computer to sleep under those conditions.
Apple's recommendation to only shut down if the computer won't be used for a length of time and sleep otherwise is based more on the convenience factor. There's nothing in there about what's better for hardware.
Anyway, different strokes etc. Not something that will ever be settled definitively one way or the other. Probably a lot depends on the luck of the draw.
Btw, Seagate's are getting a reputation lately for being pretty crappy.