What Scockery said...
I always envisioned the AAT as something of a tank intended every bit for organics as for droids. The Neimoidian's have to have some kind of standing military themselves beyond mechanical I figure, and they need a ride... and selling them to whoever wants them.
I thought, but could be wrong, that the AAT used other guns in TPM but it's hard to see. I thought they used more than the long-range cannon though.
Funny though, none of those guns really seems like it's intended for shooting people already on it... Actually most tanks don't have that ability at all, it's more a partner tank is intended to turn and one of the MG's mounted to either the turret or operated independantly by another crewman as with the .30 cal on a Sherman, were intended to spray a partner tank's hull and clear off any sticky people.
Speaking of sticky people... How awesome woudl it have been if they paid a little homage to A) Steven Spielberg, and B) one of the greatest war movies of all time, by introducing sticky bombs... you know, a bomb that sticks!? Sticky bomb! They could've completely incorporated explosives that are otherwise useless being somehow magnetically (or otherwise) stuck to not just tanks, but droids. You could've really paid homage to Lucas' best friend, and made a desperate situation look even more desperate in the toon, and less silly. Hell, you could even keep the droid coming out of the hatch and saying something dumb.