It seems to me that since RAID volumes are logical (as opposed to physical), the layout that the OS believes they have might not correspond to the actual phsyical layout.
So does defrag make sense for RAID?
Yes, defrag does still make sense for RAID. While it's true that the layout the OS sees isn't the same as the physical layout, it's monotonic, ie the virtual sectors are in the same order on the disk as they are on the array, it's just they are scattered across disks.
Also, the RAID controller will use predictive caching (if it has it) based on an understanding of the array layout, so that will work better if you have defrag.
The only time you don't need to defrag is if the underlying storage medium is random access, so don't defrag your USB key, and don't defrag an SSD.
Yes it does. No matter what RAID level you're running, you may still be affected by fragmentation (unless you're running SAN solutions like NetApp with their WAFL layout).
RAID1 is just a mirror of the fragmentation on both drives. RAID0 is just splitting fragmented files up on two drives. RAID5, 10, etc.
Yes, on a desktop PC with a single drive. Servers? Not worth the effort.