You can't access machines only by the hostname, if they are not listed in another machine's resolv.conf with corresponding ip address (I'm not talking about various naming systems (NIS, SMB, etc..), which also might have special requirements for hostname). Having in mind this, you can create whatever aliases to ips you want. The only one reason I see, why hostname and domain name are required to setup when installing Slackware is that some people might have one domain name and would like to point subdomains to each or some of the local computers.