[mtrpcic]

While I think the assessment that modern medicine is more effective, and often stands on the shoulders of these types of historically used plants, not everyone has safe access to modern medicine (either because "modern" medical treatments aren't available where they're located, lack of finances to pay for it in egregiously overpriced areas, or general mistreatment by the medical community). Using alternatives, while not as refined or high quality, is better than nothing.

[vorpalhex]

Except those alternatives will kill you dead if mishandled.

This isn't a choice between "cough medicine" and "slightly less effective cough medicine", it's a choice between a controlled and inspected product and random plants that may or may not be the right genus, dose and concentration.

[blacksmith_tb]

'Wrong Genus' seems to be giving monks and other gardeners less credit than they deserve? Sure, amateurs foraging for wild herbs could make some dangerous mistakes, but if you're intentionally growing a plant it's presumably the right one?

[Cass]

In areas so remote that modern medical treatments aren't available at all, people do often still grow their own medical plants.

If lack of finances is an issue, growing your own herb garden certainly isn't the solution. These plants aren't going to replace a $500 shot of insulin. At best you could maybe replace the occasional aspirin, dramamine, or buscopan, all of which are available over the counter at price points much lower than what it would cost to even buy a bag of quality seeds, never mind soil, fertilizer, and the garden to plant it all in. (Or the time and labor to plant and tend it).