An add-on downloader? I could agree with you in some cases, one being you join a server, download the auto laucher, now you don't have to download it from the forums to put it on your own server.
BUT
One thing I'd have to disagree with you is, lets same someone has a mod they are working on, or one mod that is private and don't want floating around out there. This is when a add-on downloader would be in the conflict of interest in the host.
Also, for that reason, unreleased mods, in a sense, could be stolen, and released in the forums.
So, I'd vote no, but end the end, that would be a server side mod, not client, so it would be up to the host to install the add-ons downuploader.