the required permalink structure, why does it need the post-name?

> Your Permalink structure should include the Post name to ensure Membership 2 is working correctly.
I have been using the membership 2 pro plugin on another site and decided to use it on another one that only uses post-id. This was a design decision from the outset to save from having unwieldy urls and it really is too late to change that now.

However, the above warning is more of a "danger" type warning (RED) without a reason why it's needed and what will be missing if it's only the post id?