I beleive Sari was used so much in the first season as a buffer to introduce the characters in a better, more drawn out manner.
The Human villains were used to allow time for the Decepticons to be introduced slowly, make the Autobots into heroes (make them look like they were unbeatable) so that when the Decepticon threat finally arrives the audience can see for themselves how much of a threat the Decepticons are and how little the Autobots can do to stop them.
That was my take at least.
Looking For: Wreckers Saga TPB Collection (with Requiem)