I know that for the most part, "one beer" won't affect me to any great extent (especially so if it's Budwater or something similar), but on the small chance I have to use my firearm while off-property, I don't want there to be anything to support the idea of a "drunk with a gun," much less avoiding legal ramifications. Since I carry all the time when I'm out and about, I never "have a brew with the guys." I might go, but I stick to soda.
And when I drink at home, it's -always- limited to one drink. While the legal ramifications are different, I don't wanna affect my aim.
And there's still the "drunk with a gun" image to avoid as well. 