These are 3 different kinds of boosters:

The Afterburner [apxthrust] just gives you a short speed boost. Nothing else fancy about this wep.

The Jump Jets [apjjet] cause your craft to 'jump' into the air. They work best from the ground, and are less effective if you're already in the air. This (somehwhat) discourages a person from repeatedly using his jets to hide from battle.

The Overboost [apzboost] is by far the coolest one, because it gives you a nice speed boost, but it makes it seem as though you are glued to the ground, and you can still maneuver pretty good while OB'ing. AI uses it well, but for humans, if you face upside down and OB, then you'll fly very high in the air (or even off the map), so that is why it isnt used.