Let me ask, are you doing any testing of potentially useful MediaWiki
extensions yourself?
I run a dozen wikis, a few of them quite large. And I am a software developer. I have put to use pretty much every significant extension to MediaWiki, and I have pushed SMW to its limits.
That it currently has limits is not, to me, a reason to ignore it.
Here is one of my public wikis, and the software I work on: http://grey.colorado.edu/emergent
I believe that if a core developer were to become excited about an extension such as SMW, they could have already scaled it to Wikipedia size. Why should we have to wait for them to become excited about something before it gets implemented? The community deserves a larger voice than that. There needs to be more rational oversight.