Unlike Paypal, John doesn't have a history of abuse, doesn't blanket ban transaction types, and you deal with the same level-headed guy every time. Until there's reason to doubt him, I'd never suggest someone not use someone who's had the opportunity to steal millions of $ in BTC and never takes it. There are plenty of alternatives, but when introducing third-party risk, you want them to have a long and spotless history -- nobody fills that as well as John (speaking as someone who offers free escrow).

With all that - John IS an easy choice, so there aren't going to be many alternatives. Competition's always good, but nobody can out-compete him AFAIK, largely because he has such an established history, and asking less in fees than him is practically impossible. So while it's not decentralized - if you're serious about securing contracts, you can't do better than having the PM page for John bookmarked.

It's for the same reason I wouldn't recommend someone use a new wallet from a team nobody's ever heard of over the established and reviewed wallet software which's been out for years. Trust and reputation's worth a fortune when conventional legal systems are too cumbersome.