It changes with any context menu selection.
For instance, lets say I'm...
-Building a playlist, and set the drop target to that list.
-While finding tracks for that list I want to play one to see if it fits, while I keep searching, so I Send To playing now.
-The drop target is not the playlist anymore, it's Playing Now. Grrrr.
The way I see it whenever you are traversing your library doing something, you may be primarily doing that one thing, so it makes sense for that one thing to be the default action. But usually when doing that one thing primarily there are other little bits to clean up, etc. so you want to do that WITHOUT having the default action change. It helps multitask. The way it is it's like it ALMOST helps multitask and be more efficient, but not quite.
Add to that the customary behavior thing. When you set something in one place, IE go to the drop target and set it to the new playlist, you are not used to having something you do elsewhere change that. It's a little unnerving.
And yes, it should be an option. Preferably a switch or a button, not buried in options.
-Nef