This has recently be fixed in Feisty:
nss-mdns (0.9-0.1) unstable; urgency=high
[...]
* Re-add a postrm to cleanup the mdns_* entries from nsswitch.conf but act on
package removal instead of purge to avoid breaking the DNS when the lib is
removed and to ensure perl is still present; closes: #406198.
[...]
** Changed in: nss-mdns (Ubuntu)
Assignee: (unassigned) => Martin Pitt
Status: Unconfirmed => Fix Released
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removing libnss-mdnss leaves system unusable
https://launchpad.net/bugs/88745
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