I see where you're going, but I guess this is where we're going to have to agree to disagree. It also depends on whether you're talking campaign or MP. I agree more for MP, not so much for campaign.
In Halo 3 MP, the BR reigns supreme. And even in the campaign, you always had a BR or Carbine - which I'm ambivalent between in campaign - on hand. There was almost never a moment without one and no reason to drop'em. I think this would get boring in Firefight. I'm looking forward to those tales of last stands, felling three hammer Chieftains using nothing but a lose leaf notebook and harsh language.
In campaign, I really think there are very few unsatisfying weapons. Sniper rifle, beam rifle, carbine, (new) pistol, (new) SMG, rockets, FRG, hammer, sword, turrets, needler, shotty, mauler. All are very satisfying in their own ways to me. And that's a lotta weapons. I don't mind being forced to mix it up with them.
Beyond that, the plasma pistol looks to have a renewed role, but one as a secondary, much as in Halo 1 and 2.
Then there's the spray and pray class: spiker, AR, plasma rifles. Not as fun. Definitely fallback material. But I suspect they'll retain that status. When I look at the balance of the list, I'm fine being pushed to use more than the BR/pistol.
To put it even more forcefully, if Firefight were built around the pistol to the same degree as MP is focused around the BR, I'd be one sad puppy. Using the full sandbox is what has me excited about it.