hedgewitch, If you don't choose class a honeybee as an organism, I can see your point. Let's apply that logic to a wider scale, in which you say the earth is itself an organism, "Gaia." To Gaia it does not matter if a single species of plant goes extinct, because millions others are present and ready to evolve into its niche. The death of an individual plant, species, or hell even 300 species is "inconsequential," as you said.
Yeti is still trying to prove that the goal of evolution is to rationally avoid death, which is fundamentally untrue, as every living being dies and 99% of species ever alive have gone extinct. However I can see trying to persuade you is useless.
My personal feeling about cannabis is that 1 million to 500,000 years ago humans learned when they ingested this particular plant it changed their consciousness in a pleasurable way, as it bound to the receptors of the endocannabinoid system already present in our nervous system. As I mentioned before, an endocannabinoid system is found in many other vertebrates and even some invertebrates, so humans were definitely not the first to get fucked up on weed, man.
Now what is the "reason" for THC? I would hypothesize along the lines of the abstract posted by 5HT2. THC, Opium, mescaline, DMT tree bark, all of these happened to evolve certain chemicals similar to neurotransmitters found in our brain. This is logical because evolution is known for its redundancy, which is why neuroscientists study rats' brains in order to better figure out human brains. With the rise of human history and consciousness and our arrogant anthropomorphizing, we assumed nature must have put them their for a reason. We cement our delusion by cultivating and potentiating these plant's chemical production through domestication and selective breeding.