>> show that it is an alias
There is no such thing as alias in the true DNS world. The thing is, for performance and reliability concerns, use the best available configuration.
>> make administrationlife easier
No at all when someone run to you and say I see an error message in my smtp log that say "CNAME lookup failure temporary". Why?
>> for the first query, after that it's cached for some time at the DNS-server you're querying
When using CNAME, dns client requires to query the A record of the associates A record starting from root servers and it requires further lookup and often may take forever depending on the level. In BIND, they incorrectly believe all answers from anywhere are to be correct and can be trust, therefore, they cache everything more than you want it to cache. This leads to several BIND exploits when hackers making up some DNS info in purpose to trick BIND to believe that everything on the Internet can be trust.
In djbdns, it only caches data from authoritative servers whose authority
can be traced to the roots. That said, djbdns trusts answers and cache it only from authoritative servers.
>> we should use IP-addresses instead of domain-names when sending email
No as that defeats the purpose of DNS in the first place.
>> Basically this discussion is about bandwidth against RFC
Unfortunately there are many design flaws in DNS protocol. Why don't you or whoever reading this thread take a few moment and read thru this short page -> http://cr.yp.to/djbdns/notes.html
and give yourself a better clue what DNS really is, and the disadvantage of using BIND.