It improves usability for you, perhaps largely because you've internalized the mental model that it instills ("there are two places to minimize applications to: one is the regular task list, and the other is this tray thing"). That doesn't mean it doesn't ruin usability for lots of other people, especially those new to computing.

You mistake effect for cause. I internalized that mental model because that UI paradigm improved usability. To clarify, the improvement in usability stems from being able to not clutter my task bar with long-lived apps that I'm not actively switching back and forth from (like music players and contact lists of IMs). In the absence of statistics supporting your case, I'm assuming it's the same for most other people who use that feature. As for new users, no one stays new forever, and it seems counter-productive to abandon increased functionality because it might confuse what will by definition always be a minority segment of users (or because some group of designers thinks so).