The issue with something like bex.io is that now we have many different exchanges, but all running on the same provider. Suppose that bex.io is able to put the best security around their systems. It still will possibly only take one vulnerability to get inside hundreds or thousands of exchanges.
Further, instead of DDoSing one or two exchanges like mtgox and btc-e but leaving the rest of the ecosystem free for trading, it only takes one massive DDoS against bex.io in order to take down hundreds or thousands of exchanges in one go.
Bad, bad, bad.
What's needed is both security and distribution. Someone else mentioned a node.js exchange product that's popped up in the last day or so, https://news.ycombinator.com/item?id=5545271. Its called Buttercoin, https://github.com/buttercoin/buttercoin.
The language its written in isn't as important as the fact that its open source. If we get an environment of several open-source exchange engines that can be hardened by their respective communities, we get security. (Yes, individual instances are susceptible to bad server configs...).
By having relatively easy to setup OSS exchange systems, we get distribution.
People have complained that mtgox is a problem because its the largest extant exchange. At least its a single point of failure that only affects btc and money on its systems. Something like bex.io is going to lead to catastrophic losses across any number of customers as soon as they start becoming a target for nefarious people.