Here's the short version of my long theory.
When you have -3,000,000 in your cities, you can't feed them by farming 5s! Let's say you lose 6k archers per raid, and you put out 10 raids per day. Losing 60k archers is not much for most large players. Me, a fairly moderate player, make 120k archers per day. Bigger players, probably, can make much much more.
10 raids=190mil food, 6mil of each.
That's comforting enough for me.
I lose some archers, yay! Less red! Less mouths for me to feed!
So the bottom line is, you just can't feed huge armies with lvl 10s.
ALSO, they give your heroes almost total 2.4mil exp per raid (Send another wave after breaker (Within 10 seconds) and get an extra 600k exp). That's crucial to leveling up your heroes, which IMO, is one of the most important aspects in the ga,e.