Someones being naughty again...

It _does_ break things if not done with very careful attention. Most of
the people using it have no clue about how it can break things and are
bewildered when it is explained to them.

For example? A router with one ATM interface going to the world with a
high MTU with an ethernet on the other side. Say you use private IP space
for links on that router. Say someone on the Internet filters traffic
from private netblocks; lots of people do. There _can_ be machines that
are completely unable to transfer data (eg. download a web page) from
another because you just broken path MTU discovery. This is not a made up
situation, this is a real example that I have had to deal with of how
using private IP space for
network interfaces used for public traffic does break things in some

Also protocols like CUSeeMe that embed IP addresses in the protocol
will break for any leaf nodes that have a private netblock address.