The major opium producing countries in the world are Australia (~30%), Italy (~30%), with the remaining being spread across the world. Afghanistan produces only 2% of the worlds opium, and yet that 2% is used to make ~98% of the worlds street heroine.
Outbidding the drug cartels, and buying the opium, is cheaper than killing the Taliban, and will win friends of the poor farmers in Afghanistan. Buying the opium, even at elevated prices, will prevent the Taliban and drug cartels from profiting off heroine production.
There is currently a shortage of medicines produced using opium (Morphine primarily). The major opiate pharmaceutical countries do not want competition, however the small amount that Afghanistan produces, would have little effect on the medicinal market.