I think it's stupid to debate about weed and work functionality when there's plenty of bankers who snort cocaine.
A fat, lazy and stupid stoner is like a fat, lazy and stupid alcoholic: a special case that doesn't justify bans.
Also I would not speak about pot at work even if it was legal, it's like talking about how you got shitfaced last saturday: dangerous and stupid.
I think that saying "let's go for a beer" is different because you don't get drunk.
Saying "let's go smoke after work" is like saying "let's go get hammered" because it has a real effect. And that's something I'd not say.
It doesn't have to be that way. You don't have to get stoned out of our mind, couch-lock and all.
I think that's only quite common due to the nature of it being illegal. Many won't use it frequently but will get themselves so far gone when they have the opportunity, just because they like the feeling.
If they care to lead professional lives, remain available to do most things during the day, whatever, it's naturally self-limiting. Some people enjoying getting hammered drunk once every now and then, but still likely find themselves drinking just one or two beers more often than not.
It's not that difficult to reach the same level with marijuana. Especially with the different strains out there - for some types, even heavy usage of one strain won't lead to the same "omg I don't want to move" feeling, or it does at very high levels but even "wow I'm high" levels still lead to an "omg let's go do something!" attitude and energy.
I don't think it will be as common as a "let's go have a smoke at the pub" situation, like with beer, but I think people will have the same attitude with it. Maybe they'll simply having a single toke or two once they get home or if they are sitting around with buddies somewhere, use that to unwind without getting blitzed. Enjoy the meal a little more, or become relaxed enough to think about something more creatively, etc.
When people have legalized access to it, and for many of them suddenly have access to much higher-quality strains (and perhaps shop proprietors will be able to explain the differences between strains), I think they'll tone it down.
Just look at how many high school kids regularly made use of alcohol simply to get as drunk as possible when they had access to it, but calmed down to a "I'll have a single beer tonight after dinner" mindsets. I'm that way with alcohol. I used to only use it to get drunk, now I'll still get drunk but that's at most once every other week for the most part, with friends. I'll enjoy one or two craft brews more often than not.