<VirtualHost fqdn.mydomain.com:80> not recommended?

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http://serverfault.com – In Apache2 documentation thay say that in <VirtualHost *:80>, the * can be replaced by IP or FQDN, but the FQDN is not recommended. Addr can be: The IP address of the virtual host; A fully qualified domain name for the IP address of the virtual host (not recommended); But why is it not recommended? If I have www.mydomain.com and webmail.mydomain.com pointing to the same IP address, why shouldn't I use the names here? (HowTos)