Single namespace? Does that even matter? Name your drives accordingly like "Movies" and "Pics & Docs" or something like that and they are essential like folders.
Logical Namespaces are incredibly important to people. That is why LVM exists. That is why RAID exists. That is why Storage Pools, Aggregates, and a host of other fancy names for multiple drives pinned together as a single volume exists. There are many reasons why people would want to buy multiple, similar drives. Limiting your storage space to a singular drive simply due to naming convention is incredibly inefficient.
I see it the exact opposite way. Suggesting that RAID is in any way a back-up solution, that is malicious. RAID is for up-time mostly in server environments (and speed in case of RAID-0) but not for back-up. The only sort-of backup would be RAID1.
I do not see anyone in this thread saying that RAID is a backup. Of all the threads on these forums where people talk about trying to set up RAID for backups, this doesn't appear to be one. The OP wants to have additional, shared, storage, and on top of that has invested in Off-Site Backups via CrashPlan. For an AT Memory and Storage thread, this is great. We may all have opinion on SHR, vs. Traditional RAID, vs. other Soft-RAIDs, but that's relatively minor vs simply having a good Storage + Backup strategy, which the OP appears to be setting up just fine.