It would be really nice if I could use mwgrep to search all instances of MediaWiki:Gadgets-definition across all wikis (for example to solve T115681), or to search MediaWiki:Common.js across all wikis to see which ones are using WikiMiniAtlas (random example). I realize the 2nd case could be accomplished with the existing functionality, but it would be a lot less efficient and you would have to analyze the results more closely. The 1st case is currently impossible since mwgrep implements a filter to look at all pages that end in .js or .css (which MediaWiki:Gadgets-definition doesn't).
For some reason the HEAD link is broken, but you can see a recent version at https://git.wikimedia.org/blob/operations%2Fpuppet/5b7895dcd5b49b385f97e99438acf837f6a1a1d8/files%2Fmisc%2Fscripts%2Fmwgrep