It's clear that dashes (-) are better than underscores (_) for search engine optimization. So people have looked for ways to make MediaWiki use -. Wikihow is one of the few MediaWiki sites that behaves this way, but unfortunately even they don't seem to be running an extension for this. I tried my luck at changing the MediaWiki core (Title.php) but I'll leave this for now. I can just tell that replacing all occurrances of '_' with '-' doesn't cut it - but probably only because this also changes the way wiki articles are stored in the database. One or two extended MySQL queries might fix that. I'll try that another day.