I kind of don't want this. Let me explain;
Here, we have some semblance of quality control. A steam workshop, we would not, nor would anyone else. It would become game banana, and all the standards we strive to achieve would be lost in the jump, surf, orange and remake maps which would be "popular" because of all the downloads and up votes. The general public doesn't understand the kind of quality we strive for, and therefore the maps that follow that will be lost.
And let's be honest, you get no decent feedback from the comments there.
Also, how often do people download custom maps? They usually get them from the server. Sharing multilayer maps wouldn't be very helpful to the developers, unless there was some way for a player to go to the workshop page of the map they're currently playing on, and it would not be useful to the players because they'll continue to get custom maps in the same way they do now.