I noticed that they added a new rule in the latest version of the tournament rules:

"Each player is obligated
to field at least one hero per game, whether or not
they begin the game embedded with a unit.
A hero must be unique for a single side (whether
playing solo or as a team), but may appear as a
unit on both sides when opponents are playing
the same faction."

Not really a huge deal, since heroes aren't that expensive and usually add special abilities that you would want anyway (like Bazooka Joe's Black Ops ability), but still interesting.