> [T]here's no real reason why ALL companies need to have their stocks compatible
I think it would be way harder to trade a basket of stocks (e.g. pairs trading, going long one and short the other) if you had to worry about mismatched settlement dates across the different stocks; it would be like trading spot against a one-day forward.
You've just defined exactly what high frequency traders do to make money. They balance all stock exchanges in order to make tiny profits on the stock differences.
I think it would be way harder to trade a basket of stocks (e.g. pairs trading, going long one and short the other) if you had to worry about mismatched settlement dates across the different stocks; it would be like trading spot against a one-day forward.