CNAME as MX, NS, SOA

[/dev/rob0]

I've seen numerous warnings against using a CNAME as MX, NS or SOA, but
no explanation about why this is wrong. I think it would be useful, just
as I use CNAMEs for all other services.

I previously was running a master and slave server on my home LAN, but
since have come to my senses. :) The master was a virtual machine (user-
mode Linux), and I would like to have changed its A record "ns.rob0.lan"
into a CNAME pointing to the slave. (This hostname is also the SOA.)

What I did instead was to change the A record to point to the IP of the
slave. So now I have 2 A records for that IP: ns and server. The reverse
zone resolves to "server.rob0.lan". Is that the right way to do this, or
should I change SOA to "server.rob0.lan."? Does the lack of reverse DNS
for the SOA matter?

And the same for NS and MX records: is there any problem with using A
records as aliases? Will nameservers care that the reverse DNS does not
match the NS hostname's A record?
-- 
  /dev/rob0 - preferred_email=i$((28*28+28))@softhome.net
  or put "not-spam" or "/dev/rob0" in Subject header to reply