I don't think this has anything to do with media players, but with, as you say, the user interface.
The user interface is Windows (and I did not try to let that be or look different, compare Foobar), so we have to deal with Windows in this case.
Now go to Explorer, and select two files, and hold ctrl and shift. Now double click ...
So the only thing I'm saying, with explicit means to make it justified
is that it is legal to double click a selection, that selection not disappearing in the mean time. If you only hold ctrl and shift, this works and this is formal Windows.
Of course ... nobody knows this combination, and after making a selection people will complain that double click deselects all. Please note that this is not much different than the questions about "can I click a file and let play start ?" where nobody realizes that it deselects again. And that you should press ctrl at the same time, (relatively) nobody knows ...
Arrggh, this is hard to explain, because all has to do with challenging for the bad things by allowing one. Maybe I can give the selection of Items in the Library Area as an example;
All in there works perfectly normal, hence by standards (apart from ctrl-a, which is not my fault really) if you leave the checkboxes Off. Now use all these beautiful standards to select your albums out of, say 1000, in order to make a Galery. Can you do that ? I bet you don't, and if you do you are dead of stress for a couple of hours afterwards. Why ? because it is not allowed for any one single time to click different than you should, or otherwise you'd loose your selections (for 1000 albums this is 2 hours of work including proper judgement).
Now, do you know what would solve this ? just not allowing the selections ! Hahaha. It would not direct people to the wrong path.
Same is with the one-click-play-please. That would be used automatically, and 95% of people (trust me) would not even know selections can be played. Or, that the click on a next track during playing, immediately would start that track for playback, nobody knowing that pressing ctrl would let the other track finish first.
Solution ? again, just don't allow it. So, click on a next track does not start the playback of it, and it needs the Play button to do so. A very easy choice everybody understand, excpect a few complaining people, eager for one-click-play not realizing the consequences for even themsleves, let alone for others.
I hope that by means of this writing you at least see that it's all thought over (even rather carefully). This does, however, not necessarily mean that I made the best choices out of things. So please keep on shooting.