Marijuana should be legalized. There's no good reason why it continues to be a crime.
That said -- and at the risk of pissing off the cool kids by sounding worshippy -- legalizing marijuana is totally and politically impractical for the Obama administration right now.
The president only has a certain amount of political capital to spend on sweeping changes in healthcare, education, energy and foreign policy. Legalization would overtake the news and the discourse and it would totally derail the administration's agenda. These are the kinds of distracting issues that routinely derailed the Clintons.
There's a right way to do this and a horribly wrong way. Legalization needs to happen quietly and carefully so as to not frighten spastic soccer moms and the like. Ending prohibition is already in progress in the states and the last thing we need is a national debate at the presidential level about this.
The president's capital should be spent on making a serious push for national healthcare and clean, safe, renewable energy -- two things that will be, shockingly enough, quite difficult to achieve. A debate about pot, with the president in the middle of it, would make achieving those goals even more difficult, and it could actually gum up the careful work happening in various state legislatures and voting booths.