The problem with JBOD is that if one drive fails, you may lose the contents of ALL the drives, depending what is laid out where. That's not good. Moreover, you INCREASE your chances of a drive failure because you have the chance of any of the drives failing - it's additive - two drives in a JBOD arrangement is twice as likely to fail, three drives is three times as likely... Oh, and unless your JBOD volume is almost empty, the loss of a hard drive guarantees you'll lose at least SOME of your data. Depending on the frequency of your backups, you may lose stuff you have not yet backed up.
Sure, RAID is not backup for your data - it's insurance against losing data to a drive failure. Yes, you still need to backup, but you are less likely to need your backups. A RAID 5 setup can endure a single drive failure without loss of data. A RAID 6 setup can survive the loss of two drives.
Oh, and with modern RAID controllers you do not need to replace a failed drive with the same make and model drive - the replacement only needs to be big enough to host the RAID content - you can replace a 100Gb drive with a 200Gb if you want (it will probably need to have the same connection - SATA / PATA / SAS - but that's all). Moreover, you can replace all the drives gradually, and when they are all replaced you can increase the size of the RAID volume. (OK, not all RAID controllers offer that feature, but plenty do).
Not sure why you think RAID is more expensive to maintain, unless you are imagining keeping a stock of hard drives to feed to the RAID? RAID may well be a little more expensive in the first place, because you are buying multiple medium size drives instead of one big one (eg: four 100Gb drives instead of one 300Gb), but it's not a huge price to pay for the safety.
So I suggest that it's wise to use a RAID setup for your images (they are worth SOMETHING to you, aren't they?). Hard drives fail - they are one of the few moving parts in a PC, and moving parts have a limited lifetime. A RAID setup (of some level > 0) will protect you from losing some data to the inevitable drive failure. Do regular backups, as well, and store some of your backups off-site, but don't think of that as being the complete solution - backups are for surviving fire or earthquake or lightning, not drive failure.
All that said, I too use external hard drives, but I only use them for backup. For primary data storage I have two RAID 5 volumes, on separate servers with separate UPS. But I'm paranoid - I also burn at least one DVD backup of the images before I wipe the CF cards...
You should be aware that most (not all) cheap external hard drive cases keep the drive spinning all of the time that they are powered up. This keeps the drive hot, and the external drive cases are usually not well ventilated. Heat will shorten the life of the hard drive, and continuous spinning will wear the bearings. If you are using external drive cases, it's a good idea to power them down when you are not using the contents.