Currently MS added the option to make an entity mailable which was a step in the right direction. However, if custom entities can be created to send emails to, they should also be available for the marketing module.
Basically, you should really be able to add those custom entity types to a marketing list.

A perfect example of this would be if you want to send mailings to people, not to their private address, but to their various business addresses based off particular functions they have.
Say I want to send a mail to Person A, once as the president of an organisation (list 1) and once as a member of some committee (list 2).
You could say these are connections, but you cannot create mailings in function of connections. You could customize something to have the info you need w/o connections, but you cannot add those custom entities to a marketing list.

You can also abuse the current contact/lead entities to accomplish something like this, but none of which is actually a correct solution.