hmmm... it doesn't work like that for me - I can just click anywhere in the Album List window, and the selection (and only the selection) goes straight to the library viewer selection playlist (no need to click the All Music root node). Or am I still not getting what you mean.. ?

I've just tried it with the album list in a separate window and embedded as a UI element - the only difference is that when you have it in a separate window, the root node is selected by default when the window is summoned, and the whole library is appended to anything already existing in the library viewer selection playlist (it's immediately overwritten as soon as the selection changes).
/edit: I guess was missing the point, and you want it to behave like the library search box (or Facets) and send the output straight to the playlist as you enter search criteria (without clicking on anything). Yes, it could be useful - it would be duplicating functionality you have elsewhere, but it would be consistent behaviour between various elements. A button to clear the filter box wouldn't go amiss either...