RAID is the best option when you don't have money to buy 2 storage units and replicate the data. Yes if you have not critical data put it on a single disk volume for fast write but the rest of your data? SHR FTW
If you have a problem with 1 disk it is normal, you can swap it and you don't loose anything. It is very unlikely that 2 disk die at the same time on your NAS.
You can have only 1 disk or 2 disks without RAID, and without you knowing one fails on you and you would wish you had RAID.
Yes it is not a backup solution, you can have your SHR or RAID volumes being uploaded to AMAZON, or other service, but if you have money for that why wouldn't you buy 6 disks and RAID it?!
Thats just common sense.