Now that the AR has Smart Scope, it actually holds its own at a considerable range but it's not a "dominant weapon" like the BR was in Halo 2, or how the DMR was in Halo Reach. Those two weapons in those games in particular were good at all ranges, so there was little point in using other weapons. In Halo 3, 4 (post-tuning) and 5, balancing is FAR better as there isn't one entirely-dominant weapon; each weapon has its own role, and its own weaknesses.
The AR in Halo 5 is much more versatile than the AR was in other games. In Halo CE, it was good but it felt like an SMG; High fire-rate, high spread, little range. In Halo 4 it was similar, except it had a lower fire rate but more firepower per round. In Halo 3 and Reach, it was next to useless so starting with it in those games was terrible, which is why everyone voted for Slayer BRs in 3 and DMRs in Reach. But in Halo 5, it works as the Assault Rifle can actually outperform those two weapons in close-mid range.