Power, availability, utility.

: the Super Pistol surrogate in CE and ODST. I blame the players
: and the need for familiarity.

I think the M6D actually helps proves the counter-point of 'need for familiarity' actually.

Most people at that point were used to pistols being back-up, last-resort weapons. Something you'd throw away or ignore as soon as you got something bigger. I know I did.

But the thing was it HAD the power, it HAD the availability, it HAD the utility. So once people discovered this suddenly it become their go-to gun, despite initially seeming puny and weak. And it REWARDED you for getting better at it.

You'll only get a tiny fraction of a second benefit (maybe) in trying to master a Carbine for closer range encounters - and then they're planning to buff the super-noob-friendly ARs and the pretty-easy-to-use BR as well so their niche is squeezed.

And what is the point in suffering with inconsistent damage/kill time of the Light Rifle when I'm already familiar with the DMR, and since everyone else also uses the DMR I have way more ammo available?

Sorry, I'm going full cynic today :P

: What the hell is the point of making an interesting gun if players won't use
: it anyway?

As a game developer you are a god. You control everything from how a weapon looks, to how it 'feels,' it's power, it's effects, it's availability, the interactions with other weapons. You give them a reason to use it and players will. Maybe because there is a particular map where it is the main power weapon, or a certain gametype where it excels, or a certain map size where it dominates.

Conversely, what is the point in making more weapons that do pretty much the same thing with slight stat differences when people don't use them anyway because you've engineered one to be more overall appealing than all the others?

: But they never made [the carbine] a starting weapon.

It's worth pointing out that in Halo 3 you could not make the Carbine a starting weapon since it didn't appear in the data of every map.

: Even when the Needle Rifle was tweaked and became "Dolphin gun:
: destroyer of worlds" in Reach I still hardly EVER encountered players
: using it when they had choice of DMR (excluding Invasion).

Availability and ease at range (DMR had higher zoom). There had by that point been a long established belief about the relative usefulness.

And excluding Invasion is exactly my point: you had to spawn with it as an Elite in later rounds and in the first round it was your only available 'power' weapon most of the time.

Give people a reason, they used it.

: The Light rifle? Psh. 343 did a pretty good job of making it basically the
: only rifle type weapon in the later stages of the campaign, and perhaps
: they did it to get players to want to use it more in MM but its not much
: different than the DMR... because that's the way the majority wants it.

Later stages of campaign? As early as level 3 - Forerunner - it was your workhorse. They also successfully made it BORING to use since you just had to spam it against Knights.

TL/DR: the design of Halo 4's loadout weapons was flawed from inception right through to implementation and I don't think these tweaks are going to change that.