Genesis 1:12 is talking about using God's creation to our good advantage and for food (see Gen 1:28-29). Smoking marijuana can have its advantages and disadvantages, and sometimes neither.. Really, that verse has little-to-nothing to do with understanding the biblical basis for smoking pot.
I want to try and take a stab at answering this. I know a lot of people probably aren't going to like this or agree, but I'm going to say it anyway..
By reading what the bible has to say about drinking alcohol and drunkenness, I think we can get a good understanding of the biblical basis for smoking pot as well. It does not say that it is wrong to have a drink..as long as you are not given over to drunkenness (written all over the bible), are not idolizing it or become consumed by alcohol (Prov 23:29-34), led astray by it (Prov 20:1), you do not serve as a stumbling block to your brother or sister (1 Cor 8:9-13 ~ Which I know is about eating, but I think these verses can relate to a lot of things), or take away from the glory of God (by what others might think or the things it might cause you to do that you wouldn't have done otherwise).
The rate and way that weed affects you is different than alcohol and it affects people differently. I know some people that can smoke it and do their Physics homework, socialize, and even study the bible. I know other people who can't be around others or get off the couch after smoking or just waste the next 5 hours of their life (Prov 15:19).
Given these things, I would say it is not a sin if you are not given over to uber highness, idolizing, being a stumbling block, not breaking the law, being led astray from God, taking from God's glory by others judging you and/or God or by you doing things that you wouldn't normally do, etc.. Remember that just like wine and strong drink is a mocker and a brawler, so is marijuana...don't fool yourself.
All the ways of a man are pure in his own eyes, but the Lord weighs the spirits (the thoughts and intents of the heart).
Prov 16:2 (AMP)