Actually nothing changes under normal conditions except that the brokers will take a cut of the profits of HFT firms by offering collocated risk managament. The next flash crash will come from a hole in a broker's risk control system. Of course, the possibility that some fund clerk does it on purpose for whatever reason is now zero and this is a big benefit of the change.
Did you ever try to get rid of 100.000 SPY´s in a millisecond between 12 a.m. and 1 p.m. Eastern Time ? You should do so - it´s fun to see the reaction in ES futures.....