Excerpt from "'Should I Buy It?' – SUROS Regime & Other Exotics"
The Armamentarium is one of two Exotic Titan chests, alongside Crest of Alpha Lupi (Titan version). Granting two benefits to max ammo / drops (Special/Heavy) and another grenade, this Exotic's all about being a walking crate of destruction. (Worth mentioning again: a 2nd grenade of any kind.)
In PVP, sometimes it's hard to pass up Exotics like Helm of Saint-14. If you don't want buff your special though, The Armamentarium is perfect. Titan Grenades are strong, so always having an extra grenade on-hand is game-changing. (Our favorites: Flashbang, Lightning, Supressor.)
In PVE, you may prefer Crest of Alpha Lupi for the quick revivals. If you simply don't have a Titan Exotic you're happy with, The Armamentarium is an excellent pick-up overall. You really can't go wrong with more ammo & 2x grenades.
Note: At the time of writing, Destiny is about to have Special Ammo nerfs, making The Armamentarium's Special Ammo perk even more valuable in PVP.