It has been a custom in Linux/Unix world that each user gets assigned a default group with the same name and the user is the only member of that group. The original justification was to allow group-shared directories with sgid flag set. With user-private default groups, users' umask could allow group-writability by default, and writing to such a group-shared directory would make new files group-writable by default. However, with POSIX ACLs such an arrangement is no longer necessary, because directories can have default ACLs for new files. Is there any reason to keep assigning user-private groups?
In an environment where you're using ACLs in addition to traditional UNIX permissions, no, probably not, other than convention. A user does (to my knowledge?) have to have a GID assigned to them, so you might as well make it restrictive, rather than assigning them to a generic "users" group.
ACLs don't replace permissions, they augment them.