DATTrader,
1) T+3 settlement is not an impediment for daytrading in cash account.
We can daytrade in a cash account AND settle T+3 the transactions.
2) "instant clearing" is not necessity for daytrading in cash accounts.
According to the rules, we can daytrade in a cash account, while the clearing is delayed, even delayed with 3 days.
3) Realtime updating the buying power number is necesity for the brokers to prevent free-riding and not loose customers.
Otherwise, if brokers stop realtime buying power updating(as you suggested), the cash accounts may buy securities for more than they can pay for with cash (and cash proceeds from prior trades) at settlement, and thus violate Regulation T at settlement.
Therefore, SEC should enforce realtime buying number updating for cash accounts, because Realtime intraday buying power updating prevents cash accounts from violating Reg.T at settlement.
The realtime buying power IS A CONSENSUS NUMBER, THAT IS UPDATED INTRADAY, WITH THE ONLY PURPOSE - TO PREVENT TRADERS FROM VIOLATING REG.T AT SETTLEMENT, THE ACTUAL CASH ACCOUNT BALLANCE REMAINS UNCHANGED DURING THE DAY BY ALL OF THE INTRADAY TRANSACTIONS, because all intraday transactions are settled 3 days later(T+3). Hence, none of the intraday transactions will affect the actual cash balance during the day, they'll afect the cash balance 3 days later - at settlement.
"Daytrading in a cash account" term is misnomer, because you can't short or use margin in a cash account.
To sum up, according to the rules, we can daytrade in a cash account with T+3 settlement and delayed clearing. Current regulations already resolve all cash account isses.
Fohat