That checkbox setting doesn't keep the drives spinning when your iMac is asleep, it just keeps the drives spinning while you're using the iMac and it's not sleeping. This keeps access to the drives fast when they're accessed during normal use. The HDD will spin down when you put your iMac to sleep, or, of course, when you shut it down.
It hasn't affected drive life for me... My current iMac is a 2011, so it would'nt count due to time. But my 2009 iMac had that setting and the drive never failed, even though it had one of the recalled HDD that Apple replaced. On our 2006 Mac Pro, the newest drive is from 2008 and it, and three others (that are older) that run continuously while the Mac is awake haven't failed.
My sister's Mirrored Drive Door Power PC G-4 has two six year old drives in it that haven't failed yet. My brother has a 2006 iMac that he never shuts down. He only puts it to sleep, and it's still running it's original HDD. I have that box unchecked on all of these Macs, since I'm the "Tech Support" for these Macs, and others.