Warrior spamming is a method for earning quick experience and prestige (I believe). People were sending armies of 500 warriors against level 10 cities, watching them die, and then doing it again. I think Warrior spamming with intent to conquer is ok (which would require larger numbers), but when you are intentionally sending small numbers to their death for the purposes of experience and prestige, it becomes an exploit.
That's my understanding, anyway.

