I think it is more secure than send money to exchange sites. They can crash or get hacked.
With an embed exchange, NXT remains in our wallet. We could buy/sell NXT for BTC or other coins exactly as exchange sites do, but with no risks...
I think you are refearing to multigateway, it's very near you can check https://nxtforum.org/nxtservices-releases/ and help testing
Unfortunately, multigateway is no trustless service.
In who must the trust reside? NXTservices?
The ones operating it. It, well not works but, ... its architecture is similar to Ripple. Distributed service relying on the trustworthiness of the set of operators.