We're forgetting the pin in any attack plan that involves anything other then choppering in air assault or stealth by whatever special operation assets Kimmet has. Still LOTS of biters there. You move a column of tanks, or armor, or choppers, or anything, you'rr going to end up getting eaten while the folks defending the walls take an easy seige. (But, now that Irwin knows about sweat, a sophisticated decoy systwm could be developed.)