Have you defragged your RAID?

Just wondering if it's a good/bad idea to defrag my Xserve RAIDs with RAID5 arrays? More specifically, has anyone tried it with Prosoft's new "Drive Genius" tool.

Apple says defragging is not necessary - something about large block sizes on HFS+. I don't buy it. At least on OSX client, defragging is analogous to new tires - feels like a brand new ride...