Hard drives are often configured to spin down when idle for a certain time. This can cause many spinups and spindowns per day. So I don't buy this at all. But I don't have supporting evidence that back up this notion.
OTOH, when the NAS spins up a single disk again, I haven't notice any additional load. Presumably, the read operation just waits until the disk is ready.
I was under the impression that this was, in fact, known to reduce drive longevity! It is done anyway in order to save power, but the reliability tradeoff is known.
No idea where I read that though, I thought it was "common knowledge" so maybe I'm wrong.