Still not sure why usability testing is important? See what a linux hobbyist found in an informal usability test on Ubuntu.
Usability testing (hereafter "UT") is getting a lot of attention in Drupal -- and rightly so IMHO. If we aim to have "100%" test coverage on Drupal's code and functionality for Drupal 7, it stands to reason we also need test "coverage" on Drupal's usability. If not, it's far too easy for bad interfaces to be developed that pass all the functional and unit tests, but fail miserably in the real world!
Jimmy Berry (aka boombatower) of GHOP fame plans to take the initial (but giant) steps in this direction in the Google Summer of code 2008.