Why is there a reliance on manually editing LocalSettings.php and uploading it to the site? Why is there not an Admin page that edits this online?
"Config file vs. GUI admin page" is a religious issue for systems in general. If you're running one just one wiki, say, as a hobby, then a GUI would probably be simpler. As the sysadmin of 15+ MediaWiki sites (config file) and 10+ WordPress sites (GUI that saves to a database) at a company, however, I have found MediaWiki's config files much easier to maintain than WordPress's settings GUI & database, to keep the settings of all our sites in sync. (With a GUI you often want a database, not a config file, to support concurrent edits by multiple admins.)
Config files have these advantages:
1. Config changes can easily be tracked, rolled back, diffed, etc., using any off-the-shelf version control system. (Even if your GUI can generate a config file to be version controlled, you don't know that its final form will exactly reflect the change you made: the "save" function might reorder lines, reformat the text, add unwanted commands that set default values, etc. This screws up diffs.)
2. "Undo" is easy, no matter how long ago you made the change. When I change settings in the WordPress GUI and click "OK" or "Save", I sometimes have to work hard to roll back those changes or even remember what they were.
3. You can put anything you want into the MediaWiki config file (arbitrary PHP code) instead of whatever limited functionality that the GUI designers believed would be useful. This is invaluable. Possibly you could factor out the simpler settings into a GUI tool.
4. Config files are easily deployed to multiple targets as part of a formal release process: e.g., rsync to your 10+ wikis. With WordPress, I pull my hair out every time an admin makes a change through the GUI on one site and doesn't document it. It can be hard to identify that change so it can be documented, version-controlled, and deployed to other sites.
5. With config files, you can use your favorite editor (emacs, vi, etc.) instead of whatever the GUI designer gives you, which means I can work faster with fewer errors using familiar tools.
The main advantage of a GUI, if it's designed VERY well, is simplicity, making administration accessible to less technical people. That's not an issue for my team (we're all technical). But I can imagine that a GUI for changing basic MediaWiki settings would be useful for some admins.
DanB