User blog comment:USARMP/Ninjas in Mass Effect/@comment-4979194-20120408223337/@comment-118213-20120408225045

Because in an era of advanced steal technology they make effective close range assassins.

On a gaming level they add diversity. They a significantly different enemy to fight and provide a challenge - forcing you to rethink traditional strategy. They force you to break from cover into the sight of the general mobs.