At the two places I've worked that offered the free lunch, there was a lot of pressure to stay in the office for the lunch, both implicit and explicit. I felt the free lunch offer was not so much a boon to my productivity but as an encroachment on my personal time.
Perhaps only coincidentally, I felt those two companies also had many problems understanding their engineering and creative employees and their needs, probably familiar to a lot of HN folks: flexible working hours, office layout, transparency, etc.