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This prevents you from selling such drops in the AH or to vendors (where you could make more money.)
Actually I find at around lvl 35+ you can start disenchanting green weapons for more than you can sell them for on the AH. Typically you could get 2 gold for selling the weapon, or 5 gold for selling the Greater Nether Essence that it disenchants to 90% of the time (or 8g for the small radiant shard for the other 10% of disenchants)
A production profession is especially profitable, as you can buy the materials you need, level your profession, then make money by selling the disenchants. A perfect deal if you ask me. You can then use the profits to buy the enchanting items that you actually need, or use half of the ones attained by disenchanting and you'll break even.
I did this with tailoring, and found it very profitable. No way I could have afforded my mount without it, and a good way to make 50 gold overnight!