It's absolutely relevant.
Marijuana may not make you violent in and of itself. But combine it with 10 or 15 drinks, a head full of testosterone, and a bad temper? Anything added on to that stuff that will alter your mental state is relevant, be it pot or something else.
Not to mention that a joint was found at the crime scene. Again, circumstantial, but relevant.
Just because you like toking up doesn't mean it's irrelevant.