My only problem with beer in the parks is that beer is usually served in short plastic cups filled nearly full with no lid, thus more likely to spill than, say, coca-cola, which is typically served in a tall cup with a lid and straw (except at AK, where there are no lids). If I had the choice, I'd rather have cola spilled on me rather than stinky beer.
Last Saturday at Epcot we were royally annoyed by a group of obnoxious drunks (four couples) on their way to the Rose & Crown pub. They were all over the walkways, spilling beer and making it difficult to get around them, yelling profanities (every other word was an f-bomb), acting like grade-A jerks. As far as I'm concerned, the problem wasn't that they were allowed to walk around with beer; the problem was that they were grade-A alcoholic jerks, who happen to make up the minority of beer drinkers.