I can sort of see that but I don't quite understand why there are different exchanges. I can't see the benefit except to those for whom it is an arbitrage opportunity.
I would also expect there to be low cost systems by which you could place simultaneous orders on all exchanges (at the cost of a slight delay in the order starting to allow them all to be posted at the same time as the furthest one.
The HFT still seems to add little real liquidity. The spreads that are shown may be narrower but the real spread seems much higher.
