The towers were originally going to be defended by the Promethean turrets seen on Midnight. Presumably, they'd have thrown in Banshees and other aerial vehicles. Given the wide open environment + prevalence of soft kill zones (which has a big gaping hole in it that lets you escape it), I'd say they were quickly thrown in since 343i couldn't think of anything. Laziness.

One thing I'd like to know is why Pelicans are rarely seen assisting ground troops. Nearly all Pelicans are armed with that powerful cannon and the laser. So why don't they help out like in Reach?

Granted, they are easily overpowered and can alienate armies, but there are ways to avoid this (note: making them do no damage like in Spartan Ops is not a solution).