Occupied port warning

During a recent training session I was reminded about a peculiar 
misbehavior that recent PostgreSQL releases exhibit when the TCP port 
they are trying to bind to is occupied:

LOG:  could not bind IPv4 socket: Address already in use
HINT:  Is another postmaster already running on port 5432? If not, wait 
a few seconds and retry.
WARNING:  could not create listen socket for "localhost"

The trainees found this behavior somewhat unuseful.  Can someone remind 
me why this is not an error?  Does any other server software behave 
this way?

-- 
Peter Eisentraut
http://developer.postgresql.org/~petere/