I've only ever used a troop 'hog once in what I've played, and that was simply getting from the C*********** to the hornets.
Combinations I have found effective, especially in Halo 2 with homing rockets, is a basic LAAV Warthog with a gunner and rocketeer. I pick it over the scorpion every time (and the combo is only available when there is also a scorpion available.
I do love that in Halo 3 I don't have the blind incentive to supplement every duel-wieldable weapon with another. They are perfectly effective by themselves (along with grenades, equipment, melee and weapon switching which becomes more convenient when single-weilding).
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