Human characters should not be a requirement. Human characters, as we've seen, have been fairly lame.
However, Human characters do not have to be crappy.
Hell, we're thinking about this all wrong. It's not human characters that suck, it's Human Children characters that suck.
I mean, look at Prime, or Animated. The adults in those series, like Fanzone, Sumdac, and even Fowler to an extent were all fairly enjoyable