If you get the first shot with a Scout Rifle at a reasonable distance, chances are that you'll win the gunfight and given the long range encounter they're ideal in, the first shot is not hard to come by and follow-up shots are easy, whereas they're much more difficult for the opponent to land. The options you gave were 3 Exotics. What happens if those Exotics are taken away? When it comes to use of Exotic vs Legendary, Hand Cannons and Pulse Rifles are predominantly Exotics, but as far as Legendary primary weapons go, I see more of them than I see of MIDA. As far as damage goes, 61 Impact SRs do 73 per headshot in PvP, meaning 3-shot headshot kills the same as all Legendary Hand Cannons excluding modifiers such as Final Round, with a higher RoF than the 81 and 94 Impact HCs, and more range on average than ALL of them. A Scout Rifle can be put to very effective use in PvP, they're just a lot less forgiving for missed shots in the CQC environment, but it isn't exactly a pipe dream to beat a Hand Cannon or Pulse Rifle.