This may explain why busy areas like Times Square aren't attacked by terrorists more often. The crowds make them tempting targets: lots of people mean lots of potential victims and subsequent media attention. But those same crowds—especially the regulars, who are always looking out on the street—make an attack harder to conceal and, therefore, to pull off. (Research project for a sociologist: Have terrorist attacks in Western cities taken place more often, or less often, in areas with lots of street vendors?)
And I'll argue that a planner should take that research challenge, not a sociologist!
*In a semi-related note, today Edward Glaeser takes issue with Jacobs' ideal density, arguing that her preferred walk-up buildings are impractical when demand for housing is as great as it is in Manhattan. Manhattan densities should be substantially higher, but too often land use regulations get in the way of taller buildings. With regard to "eyes on the street," the first few floors of a building supply them but I suspect that by the time you are on the 7th or 8th floor you can't see much down there anyway, but the higher densities put more people on the sidewalks.