Josh Holmes told the GDC that "when we started out, I was really pushing the team to go in a very sci-fi direction. I wanted to embrace the sci-fi nature of the enemies and explore concepts that were outside the box for Halo, things that you'd never seen before in the Halo sandbox."
"What we found as we were testing these weapons with players, the really deep sci-fi approach wasn't relatable. As a result, they weren't really gravitating to these weapons."
Eventually, 343 Industries had to make sure that all the weapons in the game could be understood by players immediately. For example, the scattershot is "at it's core, a shotgun," even if it does disintegrate enemies. The other weapons that made Halo 4's final roster all have immediately understandable real-world counterparts, like a sniper rifle or an assault rifle.
"One of the barriers to entry with Halo is that it is sci-fi," Holmes pointed out. "It's had a very steep learning curve that's turned off a lot of players."
To me it sounds like the catering to ‘noobs’ statement that found it's way out of GDC.
In the end, the Forerunner weapons were just variants of the Human / Elite weapons of choice – probably done mostly for balance reasons than the above…
This video below shows a very clever dude who’s figured out how to show some of these unpolished / unfinished items actually in game.