What do you mean by "need"? If you don't have an exchange you can trust, doesn't that make things worse, because you have to determine whether you can trust your counterparties and you have to secure your assets with your own resources?

Matt Levine wrote the other day: "...people think they don’t trust the traditional financial system, but in fact its most basic functionality—not forgetting where it put your money—is so reliable that people just keep assuming without evidence that the crypto system must work the same way.

Surely a big crypto exchange must have, like, systems, and lawyers, and more than one employee, so you can trust it, right? Wrong! If I ever buy Bitcoins I will keep them at Gemini, the Winklevoss twins’ exchange, just because I am absolutely sure there are two of them."