I always used the IP address and then one day a customer changed their IP scheme. I had to go to each PC that used desktop messaging and change the IP address to reach the new CallPilot server address. If I had used the FQDN, I wouldn't have had to do anything. That is the one drawback I have found. If the client doesn't ever plan to change the IP address, then I don't see any problems with using IP address.
War Eagle!!