How much? As someone who invests for the long term, I really don't see large spreads affecting me. Right now, looking at google, I see a spread of 16 cents on a $1000/share stock. If I invest in that stock, I'd probably consider anything under 1% in gains a wash, so the spread would have to be over $10 to even factor into my decision making. I don't care if the spreads go up 10x, to $1.50 on that stock, it won't affect me, yes I'll lose that extra dollar or so, but I'm trading on double digit gains/losses, if I buy at $700 and sell at $1000 I don't care about that $1.50, it doesn't materially hurt me. All that HFT does it make the stocks react faster to the news, and the cost is that the HFT people get to suck money out of the market for nothing (though make it liquid I suppose), But is that something we really need? I don't need it that liquid for my investments, and the businesses don't either.
If it was up to me I'd change the stock exchanges to process one trade per account per stock per day, all at 4pm (meaning you got the whole day to enter your trades, speed won't have an effect at all).