I find this most frustrating. We have a well maintained LocalSettings.php. It include all the operational parameters of the wiki, including the wiki name, the locations, the database user and the database password.
Yet nearly every maintenance script fails for missing parameters even though we run the scripts with --conf=LocalSettings.php. We don't really know what more we need to do when running scripts.
The ones that really confuse us are -dbuser (already supplied via $wgDBuser), -dbpass (already supplied via $wgDBpassword), and --prefix (already supplied via $wgDBprefix).
Unfortunately, we don't know what to supply for arguments like --dbgroupdefault. As far as we know there is no such thing. But the various docs don't discuss what it should be.
If this is a bug in the maintenance scripts, then please discuss it and the workarounds.