Most of the beneficial effects of cannabis come from the cannabinoids and not THC. You aren't going to be able to use that synthetic stuff for any muscular or nervous disorders and forget about using it for glaucoma. Synthetic THC does not have the same medical benefits, performs poorer for the things it can be used for, and has side-effects that cannabis doesn't, and as others have mentionned it can kill, and is physically addictive and causes withdrawal symptoms.
An interesting side-note about addictions. Most drugs are not physically addictive in their unaltered natural form but become so when you isolate one part. Unsure about opium as can't find any studies done on pure opium and not it's derivatives. Note physical addiction is not the same as mental addiction. Canabis can be mentally addictive but so can food, sex, etc...
The person who commented that cannabis sativa looks very different from cannabis indica is missing the point that both have high THC strains (pure sativas more in the mountains of the east and mexico, while indica mainly confined to middle east, hot lowlands), while hemp is grown strictly from cannabis sativa.
The real reason you can't hide hemp amongst marijuana is planting density. Spindly sativas grown for marijuana will spread out and consume up to 25 sqft for one plant. When you plant for fiber densities can reach 100 plants per sqft as you want to reduce any lateral growth and strictly encourage vertical growth as it's the long bast fibers that are used. There's absolutely no way someone can confuse the two.