The major impact will be in IRA accounts. I believe IB already does it that way. If you sell a stock, the proceeds are not available to buy something else until it clears three days later. I know that other firms let you use those proceeds immediately however.
With a cash account you can't use the proceeds from a sale for 3 days. The firms that were ignoring that were in violation. I know that Mytrack would not let me use proceeds for 3 days in a cash account. The trick was to get a margin account, not that I wanted to use the margin, I just did not want to wait 3 days. Max