That depends on how it's used. I suppose what I'm getting at here is whether your issue lies with the drug itself and/or it's social stigmas/legal status, or with the employer's right to dictate your usage of it.
Well, my feelings about weed are the same as my feelings about alcohol, tobacco, and other drugs:
-I wish they didn't exist.
-I do not want to be around anyone who is using or has recently used them.
-I think they provide only negatives to society
-I'm torn about their legality. My personal dislike of them conflicts with my socially liberal mindset
I also feel that as long as terms are set at hire, drug testing is fine.
Let me rephrase the question, if our hypothetical marijuana user was prescribed the marijuana for medical reasons, do you still feel it's the employer's right to fire them?
I think that smokable/edible medical marijuana is complete bullshit, but if you come in to work high or hungover, you should be punished.
Let's take it a step further with the alcohol and eliminate the medical aspect and look at the "socially acceptable drug" part of the equation. If prohibition was reinstated a month ago, do you think any employee who's had a drink in the last 30 days should be fired?
Should be? That depends. I would support the right for an employer to fire someone if they failed an alcohol test under prohibition. Same for weed.