Consider this, in addition to the obvious:

• It would be an epic mind fuck that democrats could not spin into a negative
• He could use it to press points about freedom and liberty
• People protesting may chill out a bit
• It would be a huge distraction
• It would be endearing to the African American and young demographics

It would be a perfect time to play this card.

I'm not sure what his actual power in this area would be but I'd say something like directing law enforcement to no longer make possession arrests, direct the FDA to remove marijuana from schedule A and commit law makers to putting forth official laws to end the prohibition of marijuana as well as the commute the sentences of non-violent offenders currently held on possession charges as well as having possession charges expunged from their criminal histories.

Also, commit funding to help with substance abuse support.