I just checked and it does work for nodes in the left panel - I usually drag and drop using the tabs above however. Say i have playlist 1, playlist 2, and library open in tabs. If playlist 1 is playing, i cant drag from library (artwork view) to playlist 1's tab, but i can drag and drop to playlist 2. I can only drag to playlist 1 if i stop it and start playing from somewhere else.
The behaviour for tabs used to be just like the nodes on the left - I could drag and drop to tabs freely in previous versions, but it has changed recently.