What gives you that impression. If I forced to guess I'd say you're right but nothing is known about how the armor abilities are acquired, selected, or changed midgame. I see two weapon loadouts in the trailer, AR+pistol and DMR + pistol, don't understand why we wouldn't be able to select between the two. Having every single gametype split into two (AR opr BR starts) in Halo 3 is retarded, I don't see them going that route again. My guess is that they feel the pistol is sufficient and you won't feel as cheated when one team grabs real long range weapons and starts donging on you, that's why most gametypes will be AR+pistol starts. Hope I'm wrong and Bungie wises up. At low levels people can select the accessible AR while at higher levels they'd be able to switch over to DMR starts. Makes way too much sense though so like AR only and BR/AR starts in Halo 3 when things could be unsegregated already by having AR+BR starts across all gametypes, they'll probably force players in one direction while having the infrastructure to do otherwise.
The AR/BR complaints are easily the biggest complaints about Halo 3 MP's gameplay, it's obviously something they should've put that as the highest priority.