I just bought my first Drobo, a 5Dt. I'm using it right now as primary storage. It's much faster loading LR and C1P than the previous external HD, even though the catalog files have always resided on the rMBP internal SSD. And, right now I'm using drives I had lying around some of which are 5 years old. I like not having to match drives as with most RAID 5 systems.
This purchase was brought about by what i thought was a failed 3TB drive. Disk Utility wouldn't even find it. It turns out that Time Machine had filled that drive to 0kb free. I removed the drive from its case, and placed it in a USB dock. This renamed the volume and I was able to format it. It's now running inside the Drobo. This is the second headache I've had with Time Machine.
Your point about the weak point being Drobo failure is well taken. Even though I have advanced replacement with DroboCare, I am now backing up to 2 rotating 4tb externals using Carbon Copy Cloner. The Drobo data volume is backing up hourly, with SafetyNet on. This way, I can read the backups directly without needing to restore from software. I can even boot from these drives as well.
I had some issues registering and activating my DroboCare. Drobo support handled it and have sent me 2 emails since to make sure everything is good. They seem to be pretty proactive these days.