"The danger of a "narco-Marxist" takeover of Colombia has always been zero. This is due to FARC's total lack of a program that appeals to any of Colombia's urban educated classes, as well as its general inability to appeal to anyone beyond a narrow band of the impoverished peasantry"
Prior to Plan Colombia, FARC ruled over a significant section of Colombia. If they could have maintained or expanded that base while pushing the rest of Colombia into state failure a different dynamic would have emerged. That in my book is a tipping point scenario that effects foreign investment, currency flows, emigration.

Agree with you on the narrowcasting nature of the ideological appeal of FARC but that doesn't mitigate their ability to disrupt, only to attract.

"It seems to me rather odd that we are subsidizing a domestic government's ties to paramilitaries whose main sustenance is the export of illegal drugs to the U.S."
The paras and FARC rely on drug money to pay for their operations ( or at least defray costs. So do many other non-state actors. Our domestic policy on drug use seriously boomerangs against the U.S. on a strategic level.