Jeff Garzik wrote:

NCQ provides for a more asynchronous flow.  It helps greatly with reads 
(of which most are, by nature, synchronous at the app level) from 
multiple threads or apps.  It helps with writes, even with write cache 
on, by allowing multiple commands to be submitted and/or retired at the 
same time.

But when writing, what is the difference between queuing multiple tagged 
writes, and sending down multiple untagged cached writes that complete 
immediately and actually hit the disk later?  Either way the host keeps 
sending writes to the disk until it's buffers are full, and the disk is 
constantly trying to commit those buffers to the media in the most 
optimal order.