Haha, nice debate I stirred here. For the record, if I could afford both the 70-200 II and the 85L I'd definitely shoot this with the 85. Obviously a zoom isn't needed in this situation. And anyone who can't shoot a portrait in focus at 7.1 with the 85L shouldn't be shooting the president.
Well, anyway, as Vincent Laforet says in his blog, there is NO reason to own the 85L if you're not shooting it between 1.2 - 2.0, as you might as well own the 70-200 II for only a few hundred $ more, with the ability to have the versatility of the zoom, much faster auto-focus, IS and pretty much equal sharpness.
There's a reason ISO 50 needs to be unlocked to be used. It's not actually ISO 50. It's ISO 100 being reduced in camera to ISO 50. The same as shooting ISO 100 and then in post bringing the exposure down a stop. I would never shoot with it.