To be honest, I dont see many other people ever sharing your situation.
Most people will just add drives and not try get them all setup in RAID 5. The extra hassle it causes is just not worth it to most people.
Sure it's working for you and your enjoying it but thats because your enjoying the technical challenge of it and trying to solve the countless issues it causes you.
Your constant posts here about new issues or questions about it just prove how much hard work it is.
There are probably several people here with huge amounts of storage, maybe as much as you, some possibly even with more but not a word from them. The main reason I'd imagine is that they are simply adding new drives to their systems and leaving them as stand alone drives. To add more space, they dont have to rent 4Tb of storage, they just add another drive.
The additional hassle you give yourself is all so you dont have to worry about 'multiple drives' but to be honest, its pretty much a void arguement.
You have one drive so you have:
d:\music
d:\films
d:\whatever
most people will just have
d: <-- is their music
e: <-- is their films
f: <-- is whatever
and so dont need the extra folder you are needing anyway.
You save yourself a drive letter but gain a folder. They just use the drive letter.
And as for spanning drives - thats not really that big an issue - you can either sort alphabetically or just by genre or whatever you need.
When they're all loaded into whatever media jukebox you use as well (guessing MC) then it becomes even more of a mute point because of the fact that MC doesn't care where the files are saved and just gives you a general interface for them.
The issue of managing your files that you try to overcome by having one big drive, most other people are just over coming by using Media Management software.
Right now you are right, 500Gb is the sweet spot it would appear for HD sizes (
http://www.pricewatch.com/hard_drives/)
a few months ago 400Gb ones were probably the sweet spot.
As for backing it all up, I think most people will consider their physical DVD collection as their permenant backup in the event the HD goes without warning. If you have warning then you just buy a new drive and copy the files over to it. The advantage of individual drives is that if one goes you are just loosing the files on that one drive and so its not that big a deal to backup.
All images and music can usually be backed up onto a single 500Gb drive.