Duckfiasco, you did not address intent. Why does one smoke a joint?
Let me try again then
I hope this isn't too lengthy.
The purpose of any practice is to ultimately reduce ego clinging and to benefit others. Any action done with this intent can theoretically be wholesome practice. Now, whether any of us here can veg out in front of the TV, read romance novels, or smoke weed with a pure, not at all self-interested intent to help others... I think that may take a very high level of attainment where one practically poops the Dharma
In the meantime, the difference with romance novels and the TV is you can put one down and turn the other off, and go practice with a clear, stable mind at any moment. Marijuana in this respect at least takes several hours to wear off. What do you do in the meantime?
It may be argued that you can practice the dharma while high. In a sense, sure, everything we do is part of the path. But in the sense of familiarization, redirecting our habit energies, that is made more difficult when we don't have all our faculties in their top form at our immediate disposal. Is your clarity such that you've already mastered keeping the mind in place in ordinary life and state of mind? My experience being stoned and around people who get stoned on a regular basis is it opens the floodgates of ideas, interesting ones but generally irrelevant. I would look at a blank piece of paper and see drawings I could do. Not active hallucinations, but just endless ideas. My husband gets ideas for music and makes songs. People laugh and apparently act spontaneously. It's very distracting.
However, my perhaps mislead opinion of what's going on is the wisdom instinct, Dharma protector, giant "No", whatever you want to call it, is effectively disabled whenever the state of mind is altered in this way. That includes alcohol, marijuana, spacing out, or generally indulging in pleasures or wallowing in suffering. The amount of effort to regain it varies among these things. The Dharma protector is what enables us to know bodhicitta from our rear end, to know when we're being realistic or indulging in our usual karmic tendencies... basically when to know we're about to try to lick honey off the sword and hopefully can be kind enough to ourselves to abstain. If you've ever done something you later regretted under the influence, or to speak idealistically, if your mind deviated one moment from the cultivation of bodhicitta and reduction of self-clinging under the influence (certainly regrettable), then you have personal experience with this. I do, at least. Why complicate things?
Whether the appearance of spontaneity or ease or perceived clarity arises while high is irrelevant, because the ideal is to be able to maintain a joyful (therefore non-clinging) mind at all times
, including death. Practicing while in a chemically altered state is silly as it doesn't benefit you 95% the other time. It's also unlikely you'll be high at the moment of your death. Put another way, we're constantly blown around in our own sober mental climate as is; I'm not sure why you would introduce yet another variable, specifically one that alters your most valuable tool in this pursuit.
Finally, I assume there was a good reason for abstaining from the wine of delusion or drugs or whatever the translations say. I base that trust in the observation that the Three Jewels have brought nothing but benefit to me so far. The Buddha says for X problem, try Y. It's worked nearly every time. So that's a major basis of my faith in this matter, also.
So there are the reasons why I personally used to drink alcohol and smoked marijuana a few times, but decided not to anymore. It's not a prescription for you, but as Dharma friends, we try to share what has helped us in the hope it will help the other, and recommend avoiding what we've seen bring personal harm to us.
Best of luck in your practice.