Abso-friggen-lutely. This would improve matchmaking drastically. I seriously hope they go this direction in the future, after seeing games like Destiny and the Division, matchmaking hoppers seem so ancient and limited now, and while I don't think open-world matchmaking makes sense for a mainline Halo game, a toggle system like yours, or the one used in PC Battlefield is absolutely an improvement over how it works now.
I just want to say that I don't hate the Assault Rifle. I think it's a good weapon, and the Halo 4 variant is better than all previous since CE (even better since TURBO) and it most definitely has it's place in Halo. I've been using it as my secondary in Halo 4 in one of my main loadouts since launch (BR/DMR/LR depending on my mood + AR + frag + hologram + dexterity + firepower). Burst fire with it is ridiculously satisfying in Halo 4, and I find it to be the most useful close-range weapon for me, even more so than the Boltshot, which I've never really liked all that much. I love hearing people cry when I am able to out-AR them at a distance using careful pulses like I see emphasized so much in the books, as crazy as that sounds.
That said, I really feel like spawning with JUST an AR is a grave mistake on 343's part. I like the BR, but I also understand that it is difficult to effectively use for the masses making it potentially frustrating for a lot of people who do like using an automatic off of spawn, and with Halo 3's and Halo 2's sandbox, I have to say that the BR is the only viable and fair starting weapon (DMR in Reach). However, this is Halo 4, and the pistol is actually quite useful (I've since adjusted my loadout to be BR + pistol + frag + hologram + dexterity + mobility) compared to the piece of garbage that were the Halo 2 AND 3 magnums. Also, the current Halo 4 BR is uber powerful, as well as the other rifles that may be littered around the map, making starting with only an AR problematic. The picture I keep seeing in my head is a BR/anyrifle user pinging me off spawn from Ring 3 to back tree by purple on Abandon. Obviously the spawns may be changed up a bit like they said, but if all I have is an AR to react there, I'm pooched. There's very little I can do as a decent player to change that circumstance if I'm fighting someone who is slightly aware of where I am.
So I say, AR primary, pistol secondary is ideal. If that proves too unbalanced (seeing as the Halo 4 pistol takes more skill than the rifles), then make a BR secondary. I just want to be effective off of spawn, without having to first hunt down something that can give me some semblance of protection at ranges greater than 20 feet.
It's worth saying that I'm against BR-only starts for a "mainstream" playlist, about as equally as I am against an AR-only start for a "classic"/"basic" playlist, mainstream or not. It is what it is. Glad to be at least having this debate again, but the real issue at hand, in my opinion is Sprint. Disabling it would make Legendary the markee playlist of Halo 4 for me.