With all due respect, correct me if I'm wrong but I don't believe you run a major exchange. There are loads of factors that need to be considered including legal, fraud, security, timezones, multi-hop settlement times (transactions that involve crossing assets) for which we are perhaps one hop and which rely on speed and precalculable latencies, etc. Basically, if anything at all to do with running and exchange seems 'obvious', you probably don't have the whole picture.

Furthermore, most of these issues are better solved in a manner that is not asset-specific or intimately connected to an asset-specific settlement network, ie. we really don't want Bitcoin to provide its own, half-assed solution for what is not a Bitcoin-specific problem.