Use of substances has been a pretty common and geographically broad aspect of spiritual practice as far back as prehistoric times, potentially. Many forms of spiritual practice involved substances, some of which are illegal today in some areas.For me personally it's that it's like a substitute, or if it works at best, it could, and often would, be used as a crutch, and as a "lazy man's way" into experiencing spiritual bliss, etc. Especially since there are other ways to achieve the same (and, from many mystic's words, much more intense) spiritual experience without the use of such things, it makes it come across as, well, redundant; almost crass and lazy. At least, to me as an outsider to the whole thing, anyway.
I'm not particularly convinced that there are other ways to achieve similar "spiritual bliss" as it's called here, for all people. For some people, they can achieve reported bliss without external substances affecting the brain. But considering how biologically diverse people tend to be, I doubt it's true for everyone. Some people report various trips or experiences or blissful states without any practice to achieve them, while others practice rigorously for mild results, while others get no results and therefore go no further in practice. There is great variation, much like how some people can barely work out and yet be physically fit, while others can work out rather aggressively and still have trouble getting in shape.
Having observed descriptions from people's reported spiritual experiences, and having observed descriptions from people's reported drug use, as well as reading some of the science that has been done so far on observing people as they experience spiritual things from meditation or prayer and the way it affects the brain, it seems likely to me that it's all biological anyway. I view people's descriptions of blissful spiritual trips in the same way that I'd view any pleasure-seeking activity.