Having network manager on the server just doesn't make sence in most cases. I'm running CenOS 6 (community rebuild of RHEL 6) on my vps without network manager and don't see any problems with this setup. I also don't know any service which would not work without network manager, I'm aware only of the opposite ones (eg. Red Hat cluster suite won't work with networkmanager). Moreover I'm quite sure that you can install RHEL, Fedora or Debian in headless mode.