They're not suppose to privatebydesign. That's why they hire us photographers because we can discern these differences for them and to give them our vision.
I know, that is exactly why they do!
You can try and hide behind veiled superiority, I have worked for some pretty discerning clients too, I know definitively none of them know the difference between a 100 shot at 8 feet and f2.8 and a 135 shot at 11 feet and f2, they know lighting, posing, framing, they demand on time results of a high enough quality to do the job, they don't give a damn how I achieve that.
But we are getting off point, as I keep saying, both lenses are very good lenses, however for me, and I would suggest the majority of users, the functionality that the 100 IS Macro L has that the 135 f2L doesn't have are more useful than the functionality that the 135 has that the 100 doesn't have.
I can well understand people buying either lens without considering the other, but if people are considering both I believe in a toss up between the two most people will get more out of the 100.
And that's your decision. I like the f/2 look and my clients don't know what exactly that is but they like it as well.
In the end, there is nothing the 100mm macro can do for portraits that the 70-200LII cannot do. I'd wouldn't buy the macro solely based on this fact. The 135L may not have IS, but it gives a unique look at f/2 that neither of these lenses can give.