There's nothing wrong with the AR, but spawning with it puts you at a disadvantage when others have picked up the BRs and are now sitting on the power positions. BRs let you break that setup and control portions of the map thereafter. The 30 seconds you spend looking for one off of the map slows the game down since you're currently too ineffective to pop out from behind a wall to ping the guy sitting in a tower. I mean, that's what a player trying to avoid a pointless death does, otherwise they'd wind up in a disadvantageous position. Why push and get killed?
It also fits the pace of Halo better where there is a mini-duel between players trying to out-BR each other. The rush for the headshot creates a solid gameplay loop that's missing from shooters today, where it's all about quick kills and run and gun, spray and pray with autos. This is fine as a compliment to the Halo sandbox, but I find it boring when it encompasses the entire game. There's no meta and no reason for shields at that point.
Maps play a large part in it. Empire is more forgiving if you don't have a BR or DMR because most of the chokepoints and sightlines are within range for the AR and Pistol. On the other hand, Truth is gigantic, which means you're at a larger disadvantage if you don't have those weapons. Once people learn the maps and the weapon and player spawns, these issues will become more apparent.
The BR/DMR weren't perfect and neither were the old games; nevertheless, Halo demands players spawn with a strong capable utility weapon, which AR starts simply cannot deliver.
I don't want the game to become "BR and nothing else" like Halo 3 was, but spawning players with an AR/Pistol to force weapon variety is a long-standing mistake Halo can't seem to rid itself of.
