Copyright assignments are harmful. They are not some sort of standard thing in open source. They would be harmful to MediaWiki.
At the risk of de-railing the discussion, I think everyone can agree that having good high quality extensions for MediaWiki is good for MediaWiki. The license an extension uses does not harm MediaWiki in any way. Nor is this discussion really aimed at proposing a re-licensing of the core. Please stay on-topic.
The question is should a WMF-funded service (and the MediaWiki community) allow 3rd party to make use of said services if they have dual-licensed code.
They are already contributing positively to MediaWiki a) they have an extension we can use that better our lives (maybe) and b) they are releasing at least a portion of their code with a permissive license which everyone can benefit from. Whether it be GPL or otherwise. True maybe we could benefit MORE, but it's not harming the core or the community.
- Olivier