Vigilance means immediacy. In any aspect of security, timeliness is next to godliness. It's much more useful to detect an attack in progress than a week, or even an hour, after it happened. It's far better to upgrade your systems in response to a vulnerability now, and not next month. Sometimes reacting late is no better than not reacting at all.
Vigilance also means preparedness.
Any detection and response team needs to know what to do when an attack occurs. Whenever processes are automated and exceptions become rarer, people forget how to react. A single organization can't provide this vigilance. A managed security monitoring service can. An MSM service can train those analysts, both in the classroom and through experience.