The concept of the middle man is an interesting topic in the study of economics, because of the argument about their relevance in business transactions. Some sees them as blessings, important intermediaries that makes transactions easy and convenient, while others sees than as an unavoidable evil, that you call to interferer, they reap and grow fat from other people's business dealings. Anyhow you look at it, whether you like them or not, they perform the important tasks of exchanges.
Because bitcoin P2P transactions are not one hundred percent reliable, especially when you're transacting with people that you don't know, using trusted exchanges becomes important because they guarantee peace of mind that most probably you won't be scammed.
It doesn't mean when you're use third party, you will be safe.
Middleman is always be a trusted, reputable and experienced, but if the middleman act unfair because the scammer bribed him, then what you can do? or maybe you have an evidence where you can prove if you're legit, but the scammer create a fake evidence in order to convince the middleman, if the middleman make a mistake by trusting the scammer, what you can do?