I'd love to see a stock market with a call auction every 15th min 24/7. It just doesn't make sense to have no trading at all during weekends and then claim that sub-millisecond execution is necessary when the market is open. The risk for the market maker shouldn't be much higher but the resources required to provide liquidity are much lower (no need to hire lots of developers to optimize your code). I wouldn't be surprised if a market like that would have tighter spreads (since it's cheaper to act as a market maker) but better stability (since an algorithm screwing up can be stopped during the 15 min interval).