Well, trust is difficult to earn. And it's often advised not to give it to anybody. We don't easily trust. But does an exchange offer a solution to this? From a certain viewpoint, it actually is making it more problematic. It adds another layer of trust. Not only are we required to trust the person we're trading with, we're now also required to trust the exchange itself. So far, it is proven with sufficient evidence that exchanges cannot be trusted however huge they are.
The issue of trust is the strong factor that we struggle with on the P2P level. Yes bitcoin in it decentralized system eliminated this aspect of third party transaction if all things are equal but just that human selfish factor that breeds distrust. But a P2P is quite successful with bitcoin transaction if both parties eliminate that distrust. For example there are some P2P local groups were with or without the third party escrow service, some parties still transact successfully without distrust which is the original bitcoin decentralized system.
Perhaps the volume of funds might be a very good reason why the use of middle man is necessary in some cases and not in others.
I for one would like to have a third party involved if I am to move a huge fund across border, just so I am sure I don't fall in the trap of paying into the wrong address that may be occasioned by a bug or malware, when copying and pasting address from my clipboard.
An exchange that is good as an intermidiary has provided jobs to people and am sure the fees won't be as high as using the centralized fiat system if we were to compare also.