About a week ago we got a new even called Scarlet Spear which was horrifically buggy. Only now after about a week can guide on Scarlet Spear actually be earnestly made. Even today there are still a ton of bugs but the event is now playable and farm-able.
The reason to farm the event is a couple of new weapons. The Basmu and the Ceti Lacera. The Basmu is worth the farm but it's certainly not one of the best primary weapons I have ever seen.
More importantly you can get the Arcanes which are normally obtained by doing Eidolon Hunting. A legendary R0 Energize will set you back for thousand Scarlet Spear points so the farm is real. Because of this, prices for Legendary Arcane such as Arcane Energize have dropped heavily so Eidolon Hunting has no real point during the event.