Why have a magnum as a secondary when the BR or DMR is more powerful? To pick off people when you can't afford time to reload your primary? I dunno, I always found the Plasma pistol to be more than helpful in many different situations since Halo 2.
I always love using two precision weapons when team shooting and flanking, but do not want to sacrifice Resupply for using Firepower.
I typically use the Magnum often in close range on enemies that are weaker than me cuz dat spam is pretty powerful, especially with this game's aim assist/hitbox. Plus I have a backup in case I get low of ammo with the BR.
Normally the Boltshot would be my choice as a secondary, but lately been using Magnum (idk kinda fun to spam it and wrack up kills). Depends on gametype/map tbh. I only use PP on big team or Crimson maps though and I'd take Dexterity over Explosives pretty much every time for dem multikills.