The "Let's legalize and tax pot" mantra would never work as it has for alcohol and tobacco. The short answer is people would grow their own in no time at all. Unlike alcohol which takes a good recipe and time to make, or tobacco which requires fairly specific soil and temperature conditions, weed requires dirt, water, and sunlight (real or artificial) to grow. And if one is experienced in hydroponics, it doesn't even need dirt.
In other words, someone who is stoned 24/7 can grow it.
Legal weed would become like tomato plants. everybody has at least one, and at harvest time everybody has too damn many tomatoes. You can't even give them away. How would the police tell the difference between a store-bought, heavily taxed spliff and some home grown pinners ?
The only real positive is that those who are going to smoke it aren't going to get laced crap off the street anymore.