Torrent idea is nice but people would disable it while playing because it would cause lag.
Server hosted mods are nice idea in theory, but I stand by my original comment. The additional traffic would generate additional costs and admins would just disable the feature.
Big source mods (Insurgency) are not downloadable from servers. You need to install them via Steam like other Source games. It only works for small mods. Even then many servers just tell you to go download the mod and come back once you're ready. Most of those mods are quite simplistic.
In addition to this ArmA servers need more room and bandwidth than other multiplayer games mentioned AFAIK. Bi did allow downloading mission files off the server so it's obviously their decision not to do the same for larger mod files.
Solutions to problems often do not scale.