Does this checkbox mean that MAPI-based RPC data will be encrypted and then encapsulated over HTTPS, resulting in double encryption?
I'm asking this question based on this understanding of terms.
The MAPI encryption that is enabled by default is between your Exchange servers and Outlook, within your LAN. Outlook Anywhere uses https over the Internet. I would not use http over the Internet, but internally there might be some benefits (eg with older clients).