I am (as I write this) backing up a failing hard drive in an iMac. This is the second HD that fails in this iMac in a 12 month period.
Now that everything is kept on a single HD until the replacement drive arrives (actually until the iMac returns from the shop, because I can't replace the damn thing - but wow that iMac is thin!!

) and I'm already rather nervous...
Hard drives crash very frequently (12-36 month lifespan) and I don't care how "cheap" they are, it starts to add up. Having things on optical media actually *saves* a lot of money in the long run and its lifetime is measured in years, not months.
Backing up to a HD is convenient (if it's cheap is debatable) but it isn't very secure. Having software on optical, music on optical one already has a sizable chunk backed up - the most essential file stuff on Dropbox and the rest (photos/movies) on a BD. So the only backup one would do is a BD backup every once in a while.
Considering how unreliable HDs are these days, I can't recommend enough backing up regularly. Just don't count on HDs for backups. You'll regret that one rainy day. I've been there, done that. Lost two hard drives in one day, just because. No reason, both pristinely kept.
Apple can keep it's MacApp Store, I value my bandwidth and appreciate high-quality backups that are automagically included with every physical purchase.
I can't help but call b.s. on your posts, sorry if i'm wrong. I get the feeling I'm not though
I'd say the giveaways for b.s. are your pointless jabs/references to the iMac you "can't replace the damn thing - but wow that iMac is thin!!

" and the supposed "lost 2 hard drives in the same day, both pristinely kept".
First, if you view your iMac as an awful computer that only has appeal in aesthetics, why would you buy an iMac over one of the many, many other computer choices available to you for less money?
Second, if you truly had 2 pristinely kept hard drives that both died in the same day, in separate computers, I'd be willing to bet everything I own you likely have an electrical issue in your house or workspace. If you lost 2 hard drives in the same day in 1 computer, then it would seem even more obvious that you have either faulty hardware in that computer or a household electrical issue.
If neither of these are the case, you have pretty awful luck my friend. Consider a visit to the witch doctor.
Assuming I'm wrong, and you have truly lost a lots of hdd's to be able to claim they're horribly unreliable, I would just ask this. What on earth are you doing to all these hard drives that are supposedly failing for you? Do you pick up your computers and just throw them around the room for fun every now and again? I ask because if you have 1 iMac that has had 2 hard drive failures in a year, it's probably faulty hardware in that computer, or faulty electrical wiring in your room, not faulty drives IMHO.
Out of the dozens of Macs and PCs I've dealt with amongst work,family and friends, I've seen maybe 4 or 5 genuinely dead HDD's in the past few years. That includes internal and external drives, and most people treat their computers like crap, particularly their laptops. I know HDD's are fragile, but seriously?
Anyhow, I used to think optical was wonderful too, until you misplace the discs, they get scratched, they can't be read properly by your particular model/firmware version disc drive even though the disc is fine, you lose your serial number for the install, or the biggest pain, your software's license only allows you to install on 1-2 computers max, so when you upgrade to a new computer you're out of installs meaning either you're totally out of luck or you have to contact the company and plead for a new authorization which you may/may not get for a few days. Been there, done that, F___ that.
So far, when it comes to backups, no form of backup has saved me more often than dropbox. I for one whole heartedly welcome lower cost, less restrictive, re-downloadable software stored on servers way more reliable than my apartment.