I just read this in the YUI Menu Moodle Doc
It follows on after a long list of near catastrophes, broken code, and despair over the YUI menu in Moodle 1.2 and later...
There are no more plans to migrate the YUI menu to Moodle 2.0. Instead, it will be replaced by a similar block that will become part of the Moodle core. A migration plan should be developed.
So why are we using the YUI menu? LOL