f/2 on a macro lens is also useful for avoiding diffraction blurring at extreme magnifications when using focus stacking. Although you really need to get to 3-4x before that would be an issue at f/2.8, at which point a 1:1 macro lens is already probably not your best choice...
But yeah, my 100mm macro is my portrait lens on the odd occasion that I need one. I got some pretty nice result's at my buddy's wedding (not the primary photographer). I've also heard them recommend specifically for portraits of babies, and babies are pretty ugly, so the shallow depth of field must be great to pretty them up.