I've seen a few questions showing up recently that are relating specifically to addons. Should they be considered off-topic? I think questions about addons that are in trunk are fine, but 3rd party plugin questions could get messy.
In this case I see nothing wrong with handling these questions, dealing with each on its own merit.
If the addon is unmaintained and/or has no documentation, then we should really not handle questions related to it. Only addons which are actively maintained, and especially those that are documented, should be supported by us (as part of the community).
Keep in mind, this is just my opinion, others, and even Stack Exchange, may disagree.
For Contrib and Trunk addons, isn't there a requirement that their documentation be sufficient before inclusion? If someone needs support for those, perhaps the docs aren't sufficient -- usually they have support threads on BA linked in the addon info.
We can easily refer to those threads, and they will generally have good content, it seems gauche to duplicate their efforts here for the sake of keeping everything on B.SE.
I say; close as off-topic but link to external support threads in the comments as a courtesy when possible.