It's a bit of a grey area as context is relevant - obviously drinking in the studio would seem way more out of place than drinking at a gig with a bar. Tho funny enough I just did a recording session on Sunday past where the drummer cracked open a can of beer as we listened through the first playback at about 1pm. And funny enough he doesn't play drums for a living and yes I do think the two are related. Not that his drinking affected his playing but rather it came across to everyone in the room as unprofessional, just as if you were in mid consultation with your doctor and he did the same thing. (side note - be wary of sessions where the record label has booked the musicians and not the producer. Not good) So my point there is that his conduct was not befitting to a professional situation. Drunk or not I know the producer would never call him again.
Yes playing music does not have the same connotations as working in an office so there is the temptation/perception that it can be treated as a night out with the lads - but to me the bottom line is that if someone is paying me to do a job then I do it to the best of my ability and that means not doing anything that will impair my performance. Of course there will always be the fully functioning alcoholic that can run rings around me onstage and I do know one in particular who is a great player all round, but I couldn't, in good faith, recommend him to sub for me or pass his number along. I know his playing, sure, but the guy/girl that's never met him doesn't so it really is a case of first impressions last. I know several stunning musicians that I simply couldn't recommend for a gig because of this and that's sad I think.