Quote from truehawk:
The PDT rule was written because the MMs would not trade with small SOES lots at their posted prices.. They would dance the fantango raising their shirts and screaming as if they saw a mouse in an attempt to avoid trading, and this made the Nasdaq stocks a lot more volitle, not because of the SOES traders but because of the MMs response to them. Companies were leaving Nas to go to the NYSE because of this MM induced volatility.
The PDT rule was a pure sop to the MMs. Their leverage and spread was not enough, they needed protection from the little guys who DID NOT lose that much. So the house had to adjust the rules of the game.
If the PDT rule was intended to protect the small investor it would have been a loss limit. Say that a new account below 25K would have an automatic stop loss of 20% of the account.
If they need new liquidity they need to eliminate this POS sop.
But the same thing can be done with a $30K account just as easily. So I don't know if the conspiracy theory holds up all that much. More than likely it was congress's reactionary response to the frenzied losses sustained by stock daytraders who made easy money during a bull market but never learned to trade a bear market and got killed.
And the rule is effectively is a stop loss. For those with account greater than $25K, they would want to manage their trading such that they don't fall below and enter into the PDT category. And those with less than the $25k required would want to adjust their trading strategy (ie swing trade) to avoid the trading ban that most brokers place on you if you're flagged as a PDT.