I agree with the comments that psychedelics may often lead to delusional thinking and the misguided opinion that one has experienced something "true" as opposed to normal everyday consciousness. However I think most people with little experience of psychedelics mistakenly categorize it as a 'lights & colors' show where magic rainbows explain the meaning of shoes. I have used psychedelics (mainly LSD) on a number of occasions in my younger days and this cliché was never the core of any of my experiences. What stood out as the most important aspect of them was a renewed perspective and my own actions in relation to others wellbeing and my reliance on permanence and material objects. It allowed me to view the effects of many of my actions from a new light and made me want to change certain negative aspects of my behaviour. Surely you could argue how stable this resolve and insight is, but for many people that questioning never comes at all. However, I see it as an incentive and neither a method or a goal in itself.
I think that psychedelics (including marijuana) may offer people from a certain background and mindset the necessary space to question certain assumptions about their lives and their own minds. It may be (note the 'may') beneficial to some people in experiencing what the mind is capable of fabricating, that there is something supramundane (although not necessarily 'true' in the buddhist sense) beyond a house and a mortgage and the occasional beer binge and cruel jokes.
If not for anything else they might be useful for realizing how ultimately unsatisfactory they become after a while when you depend on them for happiness, just like all conditioned things. Most people I've known with experience in psychedelics have eventually realized that it is impossible to cling to the experience and that there is a limit to the perspective they bring.
Surely one also has to credit the influence the psychedelics for the imapct they had on the social and creative movements in the sixties and the way they in their turn helped pave the way for a climate in society which became more fertile for "non-mainstream" religions such as buddhism. I'm sure that most people here would like to think that they would have ultimately found their way to Buddhism as they have now regardless of changes of the past but I am of the opinion that this possibility is intimately tied up with the change in society and among movements in the last five decades.
Also, yes, it may be a fabrication, but as someone pointed out earlier, what isn't a fabrication? As Thanissaro Bhikkhu explains in regards to Mindfulness of Breathing it too is a fabrication but a fabrication you learn to play with until you know how to fabricate it well enough to quiet your mind, and once the mind the calm enough and the will to stop fabricate is strong and trained enough the mind heads in the other direction - non-fabrication, i.e, the Unconditioned. I am not saying that LSD is an equally positive or quieting fabrication to Mindfulness of Breathing but perhaps the understanding of fabrication is equally important there, if it can be understood in that way.
I am not recommending that anyone take psychedelics, nor am I not recommending it. People need to make their own mistakes and realize certain things about unsatisfactoriness for themselves. I wanted however to offer some more nuanced thoughts about what could be useful about certain mistakes as opposed to others who have posted here with much experience of buddhist doctrine but little in terms of experience of psychedelics.