The ONLY difference between Legendary and Exotic Weapons are the [i]unique[/i] Exotic Perks... some are worth the Exotic Slot in your inventory... some aren't. Some Exotics are better for PvE... some for PvP... a [i]few[/i] are good for both. Most Legendaries (and to some extent... a few rares for non-Iron Banner Crucible) can perform just as well depending on the perk sets. There are a couple of Exotic MUST haves for your inventory (even if you're not going to use them all the time): Icebreaker, Red Death, Suros Regime, Plan-C, MIDA Multi-Tool... and yes... Gjallarhorn. While you DON'T need them for the Raids or Iron Banner, they are nice to have, and the Exotic perks [i]can[/i] make some activities a [i]little[/i] easier.