"The cost of mediation increases transaction costs, limiting the minimum transaction size and cutting off the possibility for small casual transactions, and there is a broader cost in the loss of ability to make non-reversible payments for non-reversible services".
Relying on a third party increases the possibility of betrayal, and finding the right third party to oversee the transaction can be an expensive hassle.
Even though BTC is not particularly practical for microtransactions right now, it may be with the introduction of the Lightning Network and offchain solutions, which still do not give any third party access to your coins during the process.