I am very much in favour of including other wikis in external links and references because I think it is a big mistake that Wikimedia projects isolate each other from the rest of the wikisphere. We do not treasure other wikis enough, behaving as though we were different from them, even better, which is not always true. Of course we cannot use another wiki that cites Wikipedia as a reference for Wikipedia, again. But we can use another wiki as a reference if it provides really good content on a subject matter which is often the case.
Regards, Jürgen.
For instance, we have Scholarpedia, which is a reliable source (no doubts about it), they have editors responsible for different fields, these editors are appointed by the Editor-in-Chief, and each article is written by a single author and by invitation only. Do we want it to be a part of WMF? I do not immediately see any net benefit.
Cheers Yaroslav