That extension is an ability of creating "rooms" on the server and playing levels (and levelsets) with others users. For example, a user creates a room other people join (or the room owner invites players for a nice treat) and then they play single levels or even full levelgroups. At one time I've suggested my good pal we can play RND with our another friendo but there are not too much available levels containing more than 2 players (if any) The server for play could cause 3-/4-player levels to be more common than ever (I would really like to play such a level[set]). Anyway, if someone didn't find a companion for multiplayer, they always would find them on the server.
Yeah, I'm fully behind this idea too, although I appreciate it might be a huge amount of work to set up.
I've actually turned a few levelsets into three player versions (to play with my children who are big R'n'D fans) by adding extra keys, exits etc. There's not much demand for this kind of thing but your proposed feature would certainly encourage people to start developing multiplayer levelsets.
The server for play could cause 3-/4-player levels to be more common than ever (I would really like to play such a level[set]). Anyway, if someone didn't find a companion for multiplayer, they always would find them on the server.