If you're worried about the photos being kept safe, you need a good backup scheme. Whether it's creating a mirror (rsync?) to an external drive or setting up a backup tape scheme that runs nightly, that should be enough to keep the data safe. It's also assurance that the data is actually backed up as opposed to available (which is what RAID does). If you screw up the photo or corrupt it, RAID will happily ruin any chance of recovery.
Will constant use kill the drive faster? I assume it'll wear the drive a little bit, but the drive failure you usually encounter comes from the fact that the platters are spinning constantly. That's whether you're using the drive or not; as long as there's power, unless power saving mode is entered on the disk or the computer is asleep, it's spinning and wearing on the drive. The real kicker is that if you let a drive sit for a couple years it may not start spinning up again, so you still lose data.
This still kind of depends on what you're doing. I mean, if you have a 64 bit OS and a turtletits of RAM, you can create a multi-gigabyte temporary drive in RAM and use that for swapping. You could also use a separate drive, but that would be for speed, not necessarily because it would kill a drive (asynch I/O in that case). But just having your own drive, if fast enough for your needs, for temp files will work fine.
What it boils down to is needing a good backup scheme. I think the drive wear thing is just overthinking it a bit.