Welcome, gns. I hope we can offer some help. I tend to cut to the chase, so let me begin by saying that you weren't educated at the time you became involved with a "functional" addict. Please read the stickies at the top of our forum. They offer a great wealth of information on what you have been dealing with.
You asked for opinions, so I'll give you mine based on my own personal experience, having been married to TWO alcoholics (we call them A's here). First, there is nothing "functional" about somebody who drinks at least a bottle of alcohol everynight in addition to "lots of pot" and other substances.
As you have discovered, this person "functions" by interacting with other addicts and tosses you a bone of companionship when he so desires. It is obvious from your post that you are not numero uno on his list of priorities. Granted, he may show up for work everyday, but as far as job performance goes, I doubt it is all that "functional."
He is now proclaiming that this is his "lifestyle." Yes, it is. And it was when you met him. It has been all along. It will continue to be that way as long as he wants it that way. See, addicts figure other people into the equation of their lives by their functionality. Can this guy hang out with me and get trashed? Can this woman let me come and go with hassling me? Can I talk this gal into jumping in bed with me? Can I manipulate whoever I need to manipulate to get my own selfish needs met?
THAT is their type of functionality and it gets worse and it progresses and it destroys everyone in its path. How much is addiction and how much is personal? Well, with regard to the relationship he had with you, it wasn't particularly personal for him, given the fact that he moved closer to his pot buddy and left you in the lurch.
You can drive yourself crazy trying to figure out the mind of an addict. He's zoned out, enjoying life, living it on his terms. As I have said before, a tiger is a tiger is a tiger. Don't expect it to morph into a zebra. He's doing what addicts do. Your trying to figure out irrational, immature, illogical behaviors will make you nuts. Keeping your focus on him will also end up in frustration.
You have to figure out what you were doing with someone who treats you like this. It doesn't sound as if he was particularly nice to you. He certainly didn't care whether or not his behavior hurt you. I would suggest you give Al-Anon a try, read some of the posts here that pique your interest, and perhaps you might want to consider some counseling. See, he has a problem; but it's HIS problem to own. You can't do anything for him until he's ready to do something for him. It sounds as if he's made a decision to remain an addict. Now it's your turn to quit trying to figure him out and place the focus on yourself.
I'm sorry you had to go through this, but we've all lived with the insanity of addiction here and it generally doesn't end happily-ever-after. However, you can live happily-ever-after, not only with yourself, but with a man who will treat you with respect and kindness.