Chkdsk is not defrag. I would not recommend defrag unless there is a very specific work load that needs to be optimized.
WAFL writes data to disk in a way that the host never sees and also does all the hard work making reads and writes faster with cache and even flash.
About the only reason I can think of to defrag on netapp is preparing to shrink a lun on a platform with no snap drive support or native filesystem shrink feature.
