I think decreasing the minimum bid/ask spread value would make it very difficult to continue doing traditional market making. I think that exchanges would then need to add some other incentives/perks to ensure liquidity providers stayed on their exchange, which starts to sound more and more like a specialist system which I'm generally against.

Of course, I don't think there is some natural right for market makers to exist. I do really like the idea of an extremely small tick size as it much more elegant than a transaction tax.