I'll give a simple example: think of an escrow service. It's clear that it provides some value, it's easy to code up and audit, and it actually needs the blockchain (so that the code is inevitably open source, and you know that that's the code you're interacting with).
It may not be fully trustless, but at least for technically savvy users it can reduce the extent of trust required sufficiently to make it sane to do OTC transactions (it's very unlikely someone would try and succeed in reorganizing the blockchain for a small transaction; the code might have backdoors, but such a simple service isn't too hard to audit - fully open source code is a tough environment for backdoors).
It may not be fully trustless, but at least for technically savvy users it can reduce the extent of trust required sufficiently to make it sane to do OTC transactions (it's very unlikely someone would try and succeed in reorganizing the blockchain for a small transaction; the code might have backdoors, but such a simple service isn't too hard to audit - fully open source code is a tough environment for backdoors).