similar topic: http://drupal.org/node/494462#new
no reason to block it. it should visitable on search engine and let users can find us.
I mean, the contact module is core, but on most sites something like a contact section (contact press department, contact webmasters) with important address information will not be indexed if u would have aliases like /contact/press or /contact/webmaster.
Is it really up to Drupal core to enforce /contact/ as a disallowed path just because core contains a module that uses /contact ?