you have to wait for the 4th TF2 server, because this will be a funmap server.
there will be no funmaps on servers 1-3, only good custom maps.
I don't like playing custom maps enough to join a custom maps server, but there are some custom maps out there, which I really enjoy playing, with turbine being one of them, labor and convoy being another two. Adding one custom map to each of the server 1-3 rotations would, in my opinion, not degrade the quality of the server. Sure, some people might leave because they don't have the map, but I am sure that most people will know that the Specialattack servers don't play shitty maps, and thus will download the map. Last time we played convoy on server2, we had full teams the entire game.
As long as the custom maps don't take over the servers, I think people will be quite happy with one custom map added to the server 1-3 rotations.