Apres event ride is pretty common on some group rides I have done - but that's when no more riding is planned until the next day. I've never ridden while drinking or ever seen it done, to my knowledge.
I did ride (once) with a guy who admitted to being stoned at the time. Funny...we had to stop at a convenience mart for Reeses' peanut butter cups. Go figure.
Apres ski beers are pretty common, too. I once told a friend that I didn't like to drink after skiing, "because it goes right to my head." He looked at me kind of funny and said "Duh. That's the whole point!"
Ah, well. Different strokes.....