@Exchange Queries - OA, EWS, OAB, etc, will be mapped to legacy.contoso.com on the E2007 servers. It's only the 2013 virtual directories that will have the specific namespaces assigned.
This scenario is leveraging layer 4 load balancing, so there is no way to distinguish the URLs being accessed by the load balancer. This means that if we are checking the health of /rpc, then all protocols go down if that /rpc fails. By having specific namespaces for each of the relevant protocols that are used by Outlook (OA, EWS, OAB, Autodiscover) we can remove the specific failed protocol from the load balancing pool, as opposed to the whole server.
When the last internal facing Exchange 2007 server is removed, you can then remove the Exchange 2007 servers from the Internet facing site and retire the legacy.contoso.com namespace.
Ross