This from the guy with the balloon fetish about the Halo 4 user interface:
When a Halo fan at Neogaf asked a question about the secondary weapon UI on screen for H4 or lack of it, Frank from 343 had this to say:
No. Not a bug. Weird emergent UI decision based on a number of factors. At first my mind was blown, because I found out about it in a review meeting internally, but then it was pointed out to me I had been playing like that for six months and never noticed - just like the grand majority of test subjects. That's the symptom, but the reason is slightly more layered.
* There are so many weapons that silhouettes become semi-meaningless. Sure you can see a plasma pistol, but by the time players get to concussion rifle versus plasma mortar, those start to lose mnemonic cohesion. You add in several geometric and semi-symmetrical forerunner weapons, DMR versus BR, increased necessary sandbox cycling in campaign, MP and SPOPs, playlist/mode cycling and several other scenarios you can fill in yourself, as well as multiple user selectable load outs, it became apparent through testing and data that the most effective method for payers to understand what their secondary weapon was - was the weapon switch button, followed surprisingly, by memory. The experience and data around those allow us to reduce UI clutter by one item. And the HUD, while in some ways more subtle In terms of saturation and brightness now, needs as much clarity and simplicitly as possible to increase immersion and enhance gameplay space without letting its animation and audio do more than increase the sense of 'place 'in the user FOV.
It's an almost invisible and very subtle change for a very subtle improvement in the experience on balance. In certain game modes, Snipers for example, it's easy to see why it doesn't matter. But ironically player load outs vastly improve player awareness about what secondaries are. Worst case scenario, you will barely notice and get used to it. Best case scenario it will streamline your experience and in a hyper edge case, save your life when an enemy Tag appears in that extra real estate.
That's an experiential description, not necessarily the design breakdown.
Just goes to show that leaving something so simple as an element of the User Interface in or out of a video game can be a quite a nuanced decision!