The extra benefit is that you are forced to have a backup

OS X rarely needs it. At least block level defrag on the OS disk. I assume you are originally a Win user as they are the users most concerned with this. I feel no benefit defragging that a good directory optimization can't just as easily give you speed wise. TTP back in the day has fried a few HD's so I trashed it. I have a copy of Drive Genius because I like Prosoft Data Rescue but I never use it. Playing with fire moving stuff around to see little to no benefit. For large data sets it makes much more sense to move data and reformat if fragmentation becomes an issue. My 2 cents. I do, on occasion, defrag XSAN volumes with snfsdefrag. But that's a different ballgame.