Parties I attend tend to have both plenty of marijuana and beer handy. People want to drink for a variety of reasons.

Sure, some people (myself included) prefer a bit of weed or a brownie over a few beers, but that's a personal preference thing.

hell, my grandmother smokes marijuana and still has a beer or two before bed.

/anecdotal evidence

I agree. Cannabis appears to be more of a stay-at-home intoxicant while alcohol is a superior social drug.