I always thought the five control-A's were a failure of imagination (never going to send binary?).  

As always, it's about history and how they got there.  What was reasonable for one group, over time was not for another.

True.  I joined BBN after the TCP/IP flag day, after 4.2BSD, and so on. When I used MH, it kept one message/file; I only know the control-a thing from MMDF.