I've also run into issues in the past where an addon didn't have "Allow updates/patches and plugins" enabled, so it wasn't possible to upload anything as a plugin for it. I don't remember which addon(s), though, as it was a long time ago.
The problem there might be that "Allow updates/patches" and "Allow plugins" are combined into one option. For my own addons, I usually have this disabled. I'd be fine with people tagging their addons as being able to plug into mine, but I don't want people to be able to attach "updates" and "patches". If there is something they think needs to be fixed in my addon, I want them reporting it to me so I can fix it in the official version.
Also:
It might a good idea to make a separate "Plug-Ins" tab in addition to the existing "Other Files" tab, and rename the "Other Files" tab to "Previous Versions". I can see many users not realizing that they can find plugins for the addon they're looking for under a tab named "Other Files", especially when Curse uses the "Other Downloads" tab on their addon pages exclusively for showing older versions.
Many people don't seem to use the plugin option. For example, on
Grid, only 15 plugins are shown there, but there are a lot more than 15 Grid plugins on WoWI.