What incentive is there to use cash to buy Bitcoins to buy marijuana when you could just use the cash to buy it? The reason to use debit or credit cards would be the convenience of not having to stop by the bank first to get cash. Bitcoins, in this case, would provide no benefit in anonymity or convenience.
I just think people who got Bitcoins might take an advantage of this.
Well apart from already having bitcoin, I would still prefer to buy lots of bitcoin for cash ONE time and then use it MANY times to buy weed. Sideeffect: you'll get more weed as btc price in usd continues to rise...