Fighters can fight; increasing military bonuses allows one to fight more, and these boosts are permanent so long as you keep the buildings around.
Negotiators can negotiate; goods production isn't quite as black-and-white to increase as military bonuses and goods inventories are constantly fluctuating, but it is possible to push up the amount produced.
The important thing to me though is that no matter what anyone tries to say, it's always possible to run out of goods. No, it's not possible to have essentially infinite goods and to be able to endlessly negotiate from the start to the end of a given battleground. Theoretically you can, sure, since negotiations can never truly be impossible, but reality says that no, you're not going to be able to negotiate forever. No matter how many buildings you have, no matter how big your Chateau is and how many quests you do, no matter how many goods you can produce in a day, you're eventually going to run out of goods if you negotiate enough. The thing with negotiating though is that once you run out of goods, you're out of goods even after attrition resets. Military bonuses don't reset. Unless a fighter ran themselves out of troops the previous day, they'll be able to hit the ground running once attrition resets unlike the negotiator who, even if it took several weeks, ran themselves out of goods because they read on the internet that they didn't have to worry about running out of goods.
Although, reading through some posts here, it sounds like what people really have a 'problem' with is negotiating providing two advancements while fighting provides one. We have to remember why Inno settled on this. It didn't really have anything to do with the capabilities of any player, and more to do with the time taken for a negotiation round along with the guaranteed cost of negotiation. Battles can be won without losses, essentially being free, and it was additionally decided that, on average, two battles could be completed in the time it took to complete one negotiation and so negotiations are worth 2 advancements. These two factors are pretty hard to reconcile, so I personally don't see any simple way to do it differently. They obviously can't both be worth 1 advancement, so what else is there?