The problem is that the impairment caused by marijuana is more mental than physical whereas the impairment caused by alcohol is more physical than mental. More people more readily notice physical impairment.
What people in those states will now quickly find is that marijuana impairs the ability to process large quantities of text. There are of course a wide variety of large quantities of text. Some is light, fanciful and with little abstraction. Some can be rather abstract, complicated and subtle. People using marijuana aren't likely to finish reading even lighter materials.
If a person only sits on a sofa watching television he might use large quantities of alcohol without noticing any impairment. That is unless he loses the remote and must cross the room to change channels. Then the physical impairment will be quite noticeable. Similarly if people never use their mental ability to process large quantities of text they might not notice that ability is impaired.
The art of debate often requires both input and output of large quantities of text. People using marijuana are not likely to engage in debate, much less win one, California notwithstanding. A rather famous debater, William F, Buckley, did advocate marijuana use at some point, but it should always be remembered that was after a rather long and successful career in debate not impaired by marijuana.
Of course alcohol also impairs mental abilities and marijuana also impairs physical ones, especially in larger quantities. It is just that different types of impairment are more obvious in each case. It is not the purpose of this article to suggest that alcohol is not harmful or should not be controlled. It is harmful and legally controlled. The point here is that marijuana is harmful and should be controlled especially because it can be so insidious.
Evidence of people performing remarkably well under the influence of various substances might be found. The likelihood is that the person had already developed considerable ability before impairment, as in the case of Buckley. It would be a mistake to assume that substance abuse can lead to better performance ever.
Part of the reason for the rise in the popularity of atheism not long ago is probably a lower reading level such as might be found with people who have impaired themselves.
Many of you will recognize another tragedy, that people who take drugs like marijuana are often seeking god. Drugs do provide illusions. If you need part of your leg amputated, drugs can provide the illusion it isn't happening. Other illusions have no good use. People might get the illusion they are reading at a higher level from drugs. They might believe they understand art better or complex and abstract and nebulous ideas better. That can readily be proven false, but it takes exposure to people who actually can read at higher levels, and we can be in short supply.