I would guess the shorter lifespan comes from cycling the spindle speed up and down more often: it's virtually always when a drive is power-cycled that it dies, rather than while it's sitting there spinning at a constant speed for days. Just like a laptop, slowing or stopping the drive will save power, but shorten the drive's life.
Lower power/heat would be appealing for a big array: a smaller/cheaper power supply per drive shelf, less cooling cost per rack, a smaller/cheaper UPS/generator etc. It's not just shaving a few % off the electricity bill for the drive itself. I get the impression a lot of big installations have a lot of "cold" data where even a 7200rpm SATA drive is excessive, but the latency of tape would still be a deal-breaker.
I wonder if they could do a bigger version, like the old Quantum Bigfoot 5.25" HDDs? Pack 10 or 20 Tb in a single unit, with slower access times - we've moved the other way, to 3.5" and now 2.5" drives to get faster and faster access times, but in some cases a slower bulk HDD is just what the doctor ordered.