If you are looking for a reason to have Diplomacy in the game then I will try to give you a few. They may not all be implemented currently but they are slated to be put in eventually.
I will try to describe it in the conventional MMO platform.
Diplomacy is more of a Support sphere used to augment the other spheres. It helps the Adventurer sphere by unlocking certain areas of a dungeon that you would otherwise not be able to enter and thus be able to kill more Oober boss mobs for better Phat Lewtz. High level diplomats are REQUIRED in order to help Adventurers obtain a flying mount for lvl 40+ Adventurers(as well as needing high lvl crafters). You want a flying mount? Then better grind out some diplomacy levels or make sure you have a best buddy who is a high lvl diplomat, OR fork over Mucho Gold as payment for the diplomats services.
Diplomats are/will be used for long duration(3+ hours) adventurer and crafter buffs. Find me an adventurer class that can buff do that. Most you get from Adventurers will be only 60 minutes.
The best items in the game will come from the participation of all 3 spheres.
You want an example? Diplomat parleys for a cool rare recipe. Gives that recipe to a crafter. Crafter sends a group of adventurers out to kill Boss Mob Whatever for a rare crafting component. Crafter takes rare component from Boss to make Oober Sword of Blot-Out-The-Sun.
Want a guild hall? Diplomats are required to make a guild hall AND to keep a guild hall. Want the fastest mount in the game(not including the Flying mounts)? The T5 diplomacy mount, with the diplomacy horse shoes and barding, will make it faster than any other land based mount in the game.
Diplomats will also be able to "own" a portion of land which will generate "taxes" that will become a source of income for the diplomats.
Diplomats will be able to do quests that will decrese the price of placing items up for sale on the Auction House. Diplomats can become Smugglers and traffic in illegal goods in order to become rich BUT also run the risk of getting caught and paying steep fines and losing faction. You want to make up your own quests and give them out to your friends? Need to be a diplomat in order to do that.
There is sooooo much more that will be available to Diplomats but I would suggest that you head over to
www.VGtact.com and do a bit of research to find them all.
Some people actually love the Diplomacy Sphere enough that they just work that one sphere. Other folks play all three spheres. Diplomacy isn't required to play Vanguard so if it seems like it is useless to you then by all means, don't do any diplomacy.
What strikes me funny is that the OP states that the Diplomacy sphere is worthless yet he knows absolutely nothing about the sphere other than what he has come across in the beginning diplomacy tutorial in Tursh. My suggestion is that you learn a fair amount of information regarding a particular topic before you deem it as being worthless.