The macro lets you focus closer, thus gaining greater magnification. With the 100mm macro at minimum focusing distance (about 200mm), your field is the size of your image sensor. Typical lenses don't allow you to get anywhere near that much magnification. The 70-200 f/2.8 IS can't focus closer than about 1400mm.


If you don't want to focus closer than typical lenses do, there is no reason at all to get the macro. The idea is to get pictures of tiny things, not to get more background blur.


It is true that when you magnify you get very diffuse background blur and narrow dof. But if you take a picture- framed the same- with two different 100mm lenses, one macro and one not, there is no difference.