It would make me mad.
I like to be able to use it both expanded and un-expanded. If you changed it, there'd be no way to switch to the "Audio" part of the tree without expanding the Audio part of the tree.
I'm not suggesting that the ability to collapse be removed, only that it is expanded by default and the second click collapses the menu.
Is there an argument for why clicking twice is so difficult?
I can click the button twice, or from now on I'll simply click the triangle once to expand in one action. My comment (and the fact that I overlooked it) is simply to point out that the current behavior may not be as intuitive and self-documenting to new user of this application.
There may be a solution that would make us both happy. It comes in two parts:
1. The button itself could be modified to better to indicate that it has two hot regions. The button part and the "expand" part. For instance, Outlook 2003 has a "New" button with an arrow next to it. When you hover over the button, you then can see that the button has two parts... a vertical line separates the word "New" from the triangle. If you then tap the triangle (or any part of the right side of the button to the right of the vertical line,) you get an additional menu of items that you can create as "new." This vertical line can appear when you hover over the button, just as now how the button gets more colorful in the Noire theme when you hover over it.
2. The entire right side of the button (on, and under the triangle) should be the hot area to expand the button contents. Right now, you have to tap on the triangle to expand it in one shot. Tapping below the triangle simply presses the button and does not behave as though you tapped on the triangle itself.
Would this be a good compromise?