Ah, yes, 'resource limitations and time constraints'. Because padding the level with redundant button pushing was far more important than polishing the vehicle segment.
It really feels more like bad time management than anything else.

: 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.

Now that's more like it.

: 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?

Because they don't HAVE the laser. Not in previous games.
In Halo 2, we saw Pelicans attack the Scarab with rockets. Didn't do much. In Halo 3, the Pelican that saved you on the first level actually takes out Phantoms with its rockets. Not sure if they were used elsewhere, like the last Scarab fight.
In Reach, I can't think of any instance where a Pelican used its chin gun, though there were certainly some missed opportunities. Hell, Falcons will actually engage ground targets, though it's often blink-and-you'll-miss-it.
In 4... Hell if I know. Haven't played it enough to remember one way or the other.

: 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).

Try having them engage enemies the player isn't fighting. Like in Long Night of Solace, when you reach the ground facility and a Pelican is dropping off squads of Marines. It would've been nice if the Pelican had hung around and buzzed the beachline when the Covenant show back up.