Buy it if you intend to use it specifically at f/2. It is capable of creating magic images. The rest is pretty well covered by your 70-200 which presumably also has a very important image stabiliser feature.
I'm considering one for myself, but notably my zoom is f/4. I also have 100mm macro L but they create very very different rendering.
Don't forget there is a more modern Sigma f/1.8 version out there and it is supposed to be even sharper, not that the old Canon is lacking in any way.