Apparently, screaming at the disks introduces unexpected vibration which the drives react to with slowing down/pausing I/Os for a short period of time, resuming at full speed shortly afterwards.
This is a pretty graphic example of what forms unwantend vibration can take (other sources may include e.g. worn out ball bearings in fans or saber saws) and how it affects disk performance. And those sources of slowdown also might be completely unnoticed unless you have performance data of your I/O latency with sufficient resolution.
Strong plus points again for DTrace and the AJAX performance data interface, that's something other vendors probably can't offer in the same quality and most likely not free of charge in their SAN and NAS products.