The worlds feature is part of Tiled and as such could be considered under continuous development, though for Tiled 1.3 only some small improvements have been made.
Making the worlds editable in Tiled was part of the initial plan for 1.2. However, I didn’t get around to it. It would be a significant effort and those who asked for this feature were not very interested in it since their maps were in a grid and they relied on the pattern matching approach.
However, there are of course several projects that would benefit from just being able to drag a map into the world and then drag it the right place, and it’s something I’d eventually still like to see.
I wonder if this would make sense as a new tool or as a new “WorldEditor” (next to “MapEditor” and “TilesetEditor”). I think the latter would suit better with the file-based undo system we have right now and it would avoid potential complexity when adding it as a new tool to the “MapEditor”.
Let me know if you’re interested in helping out with this feature!