I was browsing some site, that was initially used for Command & Conquer mods, but it seems they have expanded their site to now include WoW Addons as well.
Of course there's a lot of addons that are hosted here that is also hosted on their site, some of them even have an wowui incgamers website, although it is likely not noteworthy as most of them have the correct author website.
They have some sort of downloader that you have to use. I can't tell if they pull downloads directly from here or their server. Or if authors themselves have opted in or not. There aren't any by Clad or Ammo (two authors I remember who won't allow it) and they only have a select variety of mods. So, who knows.
(Sorry, I'm just jaded by the recent attempts by others to upload mods to various sites without consent.)
http://files.filefront.com is also "hosting" many addons. It seems to be mostly a mirror site so to speak. But there are both user ratings and comment sections for each file. Very unorganized. I feel somewhat sorry for the users who download from there. They will likely get outdated versions with no support at all - that's until they find the "right" site for the addon of course.
Well, this isn't a problem of open source itself since the same could happen to precompiled programs.
Thinking about it I have never been asked by someone if they are allowed to host my addons (only curse once asked if I will update again once I'm fine with the code).
OK, it would be quite annoying if every creator of an UI compilation would ask and I don't have a problem with my addons being in compilations as long as their creators don't claim my addons as their work. If problems arise the compilation's creator can redirect the users to the original addon page and all is fine.
But other pages hosting my addons without asking (resulting in me not even knowing that my addons are hosted there) can result in pissed users that aren't getting support for their questions since I don't know about them (and even if I would I wouldn't be answering them there...). Only exception for this are chinese/korean pages as I wouldn't be able to answer there anyways ;) still I hope they named me as the author.
But I think the main problem is that most of the people creating such pages don't know that they aren't allowed to host these addons without explicit permission due to copyright.