Invoking a hot key to invoke an AppleScript that uses display dialog to tell me my boot volume is not only a fancy way of opening another window, but even worse, it squanders a precious hot key. (Besides, without a third-party utility, you can't assign hot keys to items in the Script menu.)
Putting something in the Dock wastes space that is even more precious than Toolbar space. My Dock is reserved for things I use all the time.
As it is, the (four-letter) name of the startup volume takes little room on a part of the Finder windows that I can't hide anyway and have little other use for.
It's important to point out, though, that telling me which volume I booted from is only a side effect of its real purpose, which is to let me quickly focus a Finder window to the top level. Clicking on this item in the toolbar puts that window in column view (because I've configured it that / always opens in column view) with all the volumes listed in the left-most column. Since I didn't use the sidebar, this is how I navigate to places that aren't near the current location. (This behavior depends on the sidebar being hidden. It it's visible, you don't see the column containing volumes, presumably because they're already listed in the sidebar.)
The fact that the toolbar item also gives me the name of the boot volume with zero additional effort is a pleasant side effect.
I started doing this under Jaguar. Leopard wouldn't let me hide the sidebar, so I learned to live with it. Snow Leopard will once again let me hide the sidebar, but I've kinda gotten used to it now, and besides the screen on this machine is wider and I don't begrudge the space so much anymore.
