It is not the purpose of the icon to tell me what it does but rather, what it is.
I'm confused. I agree with what you're saying here, but I think you're saying the icons in Halo 4 have to be needlessly abstract. And I don't agree with your comparison of icons of AAs and weapons because they're two entirely different things. Isn't it just easier for icons of AAs – and only AAs, because they do not have the bells and whistles of guns, and because they only serve one function – to tell me what they do? Telling me what they do tells me what they
are.
It's easier for me to learn that this
is the symbol for jetpack than it is for me to learn that this
is the symbol for...whatever. I have no idea.
Of course, once we memorize what each icon is for, you have it nailed down and you won't need the icons to tell you what they do. This was the way it was in Halo 3 albeit with simpler designs. For Reach, Bungie decided it was best to rework the icons for the new AAs because it was hard for players to memorize each icon for each piece of equipment in Halo 3. And in Reach it was much easier to memorize them. Yes, the job of an icon is to tell what something is, but in this case, it's
easier to convey what it does – and in doing so, what it is.
Edit: Also, further clarification on one point. You can't have an icon for the BR or DMR tell you everything it does because the icon would become too cluttered. Why would you want an icon for the BR give its silhouette, that it has a 2x scope, kills in five shots to the head, and its best at medium to close range? That would be too cluttered. For AAs it's easier because they only do one thing.