Perhaps you are not familiar with industrial hemp. While it belongs to the cannabis family it is very low in THC but its value lies in the physical properties and low maintenance. The entire plant is used, so it does not naturally become a source of CO2 or CH4.
It is useless as a recreational drug, but the plant (there are several varieties of hemp) has an extremely long fiber which lends itself to all kinds of purposes.
I was going to make a list of the industries which can use hemp, but the list is too long, so I'll refer to some sites which explain the history and use of hemp in the US.
I am sure you knew that presidents Washington, Adams, and Jefferson all grew hemp and experimented with several varieties.
Few people know that the first draft of the Declaration of Independence was (is) written on hemp paper and has endured
and from wiki,
http://en.wikipedia.org/wiki/Hemp
Uses
1.1 Food
1.2 Fiber
1.3 Building material
1.4 Plastic and composite materials
1.5 Paper
1.6 Jewelry
1.7 Cordage
1.8 Animal bedding
1.9 Water and soil purification
1.10 Weed control
1.11 Fuel
Producers
3.1 Australia
3.2 Canada
3.3 France
3.4 Russia
3.5 United Kingdom
3.6 United States
I can add that hemp lends itself to the production of the finest quality cheap clothing and fabrics. It can be spun like wool and is much stronger than cotton.
Note that there is no mention of medicinal or recreational use, because industrial Cannabis is not Sativa or any of the other high THC producing varieties. Hemp grows in very poor soil, requires very little water and its roots are a favorite food for ground worms, which improve soil conditions.
All by all, it is a demonstrably versatile farm crop and to treat it as a weed is an insult to nature's gift of this remarkable plant.