While the wind is always blowing someplace it might not be at the wind farm location when there is a demand. I've driven by those vast wind farms in SW Minnesota as well as out here on the SD tundra on blistering hot summer days and haven't seen nary a blade turning while the air conditioners in Minneapolis are panting for power. I'm sure the windmills are also humming along at 2am when a cool early summer breeze drops temps to the 60's and the only lights on are street lights.
Face it windmills just aren't cost efficient as a means of generating electric power.
Your conclusion doesn't follow from your argument. Cost effective is a measure of the price of the windmill and supporting equipment vs the value of the electricity. Using that measure and allowing the 1.8 penny ptc, they are quite cost effective.