I recently booked myself in United economy to fly back to the United States from Buenos Aires for Christmas. Why would I torture myself in economy? Well, a few reasons...

It's the holidays. Holidays = not much premium cabin award space, and economy cash tickets are pricey.
There aren't too many Business Class product options between Buenos Aires and the United States, and the one that has the most award space tends to be United. United's Business Class is OK, but nothing to write home about. Sometimes I don't feel like using more miles for it.