At the very minimum you get MPEx signed receipts, so you can clearly prove what happens, rather than the situation here where the site does something,
Ehm... Suppose there is a connection problem and you DIDN'T get a signed receipt.
Now what, can you prove that you HAVEN'T got it? You don't even know whether server registered your trade or not.
Do you know a story about Byzantine Generals? Look it up.
BTW trade via colored coins will be Byzantine fault tolerant in some way (due to use of Bitcoin protocol which is Byzantine fault tolerant), while simplistic HTTP-based trade won't be, ever. (Not that it matters that much, but it's nice.)
You miss the point. The Byzantine generals situation would be akin to completely cut. communication, no technology will help here. And colored coins are still vulnerable to doublespend attacks if not done properly. GPG has years of development and fixes behind it and allows for immediate verification without waiting for blochchain confirmations.
In case of MPEx, you are supposed to issue STAT call to verify your order whereabouts, you get signed response for that as well. Alternatively, all orders are shown in public market depth data and all trades go to twitter.. So this problem is practically solved, you do have tools to get the information beyond doubt. It will be better to ask such transparency from other exchanges instead of waiting for some colored coins wonder that will supposedly solve all problems.