By default Liferay's User Management Service does not allow numeric ids. However it can be allowed as long as one limitation is understood: the numeric id cannot match the id of any entry in the Group table. This limitation is thus technical and while it's challenging to solve it, it should be explored.
Numeric ids are often used in cases where there is an external users repository where users already have assigned IDs. For example, in an intranet scenario there might be a Users Directory (such as Active Directory or other LDAP servers) where each user has a numeric ID. If one of those IDs happen to match a group id, then the user is left out (or requires a custom work around). The chances of conflict increases with the number of users. In cases with tens of thousands of users or more it's not that unlikely.