Why?! It's so sexy and matches the art direction of the game. It would have been perfect! It's so sexy it makes all the Covenant weapons all wet and horny. I can say nothing more lest I cause you to orgasm.
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It was going to be in the game, the rest of the weapons on this page of 'The Art of Halo 4' are almost identical to their in-game counterparts. My only guess is (Modders found animations for) duel wielding going to originally be implemented but removed wether because of time constraints or the fact there aren't enough weapons to do this with. Either way, in guessing that's what it would look like in game, or close to it. My MegaBloks SMG looks similar. That OP pic looks very nice though, kinda like a Hybrid of ODST and 3.