We recommend enabling implicit XWikiAllGroup on production instances that have large numbers of users, see https://www.xwiki.org/xwiki/bin/view/Documentation/AdminGuide/Performances/#HLargenumberofusers
We should enable this configuration by default.
Currently we have some issues when deleting and restoring some users: they lose their groups and rights (see XWIKI-11624). Having an implicit group would prevent the need to manually add the restored user to the default XWikiAllGroup in order to have some rights.
Also we should protect the default implicit group and treat it special. We shouldn't allow its deletion, see XWIKI-15413, we could display it somehow special in the Groups list (see a possible proposal https://design.xwiki.org/xwiki/bin/download/Proposal/Users/ImplicitUsers/WebHome/groups.png) or even in Rights (see a possible proposal https://design.xwiki.org/xwiki/bin/download/Proposal/Users/ImplicitUsers/WebHome/rightsSmall.png). Adding and removing users should refresh and always keep up to date the status of the implicit XWikiAllGroup group, or whatever its name is defined in #xwiki.cfg#.
Some related ideas about this issue can be found on https://design.xwiki.org/xwiki/bin/view/Proposal/Users/ImplicitUsers/