From my understanding a control sequence is ended by any non-alphabetic character such that
\mycsA is one token, but
\mycs1 is two tokens. This means things like starred commands
\mycs* are actually two tokens with the
* being the first "argument" to
\mycs (even when
\mycs isn't defined as taking an argument. This seems to be more confusing than defining a much smaller set of characters that end a control sequence (e.g., white space). What is the advantage of TeX behaving the way it was designed?
EDIT I realized from David's answer, that my focus on terminating characters is incorrect, and I am more interested in the advantages/disadvantages of only allowing a small set of characters to be easily used in command sequences.