Drobo seems to be a NAS storage device thats fairly flexible and low cost, it wouldn't be viable for offsite storage and rotating backups offsite daily.

Moving the drives in and out of the device would be sure death to the drives in relatively short order (We spent 2 years working with Disk to Disk to Disk backup systems before finally moving to Disk to Disk to Tape for reliable offsites. (We had a 75% failure rate in recovery in our test cases that the drives would fail in the long run.)

I'm currently running centralized backups for our corporate headquarters off a single LTO3 drive which is backing up an average of 2 tapes weekly with a 2nd LTO3 library at our datacenter doing the same. In the three years I've been running these solutions (2 years running Disk to Disk to Disk) I have had 90% fewer problems with D2D2T than I did D2D2D...

That said, you can cost justify the extra expense to the suits that matter by pointing out higher reliability (I'd find whitepapers on this, should be available from any vendor who sells this crap) and the total cost of having the data backed up vs having to send in drives for repair/recovery with a system like OnTrack and the downtime involved in said solution.