I think the "Talk" or "Discussion" pages on MediaWiki sites are mainly intended to discuss the content of their respective pages, whereas I assume this proposal is meant for general questions about MediaWiki sites/software/server itself as a whole (judging by the example questions – although I see that the proposal's description doesn't necessarily specify that, so I could be wrong).
Your point about the effectiveness of Talk pages vs. Q&A sites is certainly valid and I'm sure many others agree – for example a site running MediaWiki could host "meta" pages about the site/software/server itself, with Talk/Discussion pages where users can ask questions about editing or maintenance (take Wikipedia's Help namespace, for example). But it is a wiki after all, and as you mentioned, is available for (usually) anyone to edit; and as a consequence, that meta-content might become so vast that it becomes difficult to find the best answers among that content (take Wikipedia's Help namespace, for example)...or for the best answers to get edited out by other users. Sometimes if one has a specific question, it would be nice if the best individual answers could be voted up to the top, instead of potentially getting edited or buried among other content (as is possible in a wiki); so also valid is that some may find the Stack Exchange Q&A format more effective in that regard. Opinions!