From time to time, I have cause to create users with no homedirs, but in those cases, I set the homedir to /.

In your case it sounds like they do have a home dir set (like /export/home/username etc) but the directory is missing? If so, and if these home dirs are not some kind of broken attempt to create users with no home directory, then it should be fine to create them.

If you do though, I'd suggest skipping any users that have home dir paths on your root filesystem. This protects you from teh user creating a large file and interfering eith the system's functions by filling up / or some other important filesystem used by the OS.