If you use ncat instead of nc your command line works fine with multiple connections but (as you pointed out) without -p.
ncat -l 2000 -k -c 'xargs -n1 echo'
ncat is available at http://nmap.org/ncat/.
P.S. with the original the Hobbit’s netcat (nc) the -c flag is not supported.
Update: -k (–keep-open) is now required to handle multiple connections.