It seems that the list=users and list=allusers have some overlap in functionality and I believe they should be a single module.
I'm not quite sure about this though so I'm going to start this bug as unconfirmed.
Version: unspecified
Severity: enhancement
The real difference is in how the users to be listed are selected: list=users takes an explicit list of usernames, while list=allusers lists all users by default and filters the list using aufrom/auto/auprefix/audir/augroup/auexcludegroup/aurights/aulimit/auwitheditsonly/auactiveusers. It's a little like the difference between using prop=info&titles=... and prop=info&generator=allpages.
They could possibly share a class, much as list=backlinks, list=embeddedin, and list=imageusage all use the same 'ApiQueryBacklinks' class, but I'm not sure how much sense it would be to try to put both a parameter like ususers and parameters like aufrom/auto/auprefix/audir/augroup/auexcludegroup/aurights/aulimit/auwitheditsonly/auactiveusers on the same user-facing module.