Because their capital base is less stable, they will be more prone to stop making markets precisely when you need them most. That will probably make extreme volatility events like flash crashes much more likely.
This is already happening today to some extent:
[1] https://www.bloomberg.com/view/articles/2016-10-07/flash-cra...
[2] https://www.bloomberg.com/view/articles/2015-06-03/people-ar...
'In the new system, the market makers are computers, and when things get hairy they just stop buying pounds and walk away with their computer hands in their computer pockets, whistling a jaunty tune out of their computer speakers.'