I generally agree with your major thesis which is that the laws need to be consistent and fair. Clearly the majority of Americans see no harm in an occasional toke, and the majority indeed have tried MJ. That in itself argues MJ should be made legal.

The one argument that the police might make against legalization is that it deprives them of a useful tool -- an immoral tool, but nonetheless vital -- for getting scumbags off the street.

Example: The cops stop a guy who they basically know committed a murder or a rape or some other violent crime, but they don't have the evidence to hold him. He'd call his lawyer, the lawyer would come in and make sure he's held for no more than the maximum number of hours, then he'd be freed and nothing gained by anyone. But if they can nail him on possession, they at least can get him off the streets for some days, weeks, or months, while they gather the evidence to really put him away.

Not really the spirit of the law, but it works. That said, I'd rather we focus more resources on the few sociopaths and violent types out there, and clear out the over-crowded prisons.