The 135 f/2 can certainly be used for macro photography, just add an extension tube. Even more extreme is adding a TC + an extension tube! It autofocuses faster than the 100L (such as for sports or faster portraiture), has better bokeh, and isn't really any less sharp.
In my opinion, the only downsides to the 135 are its focal length and lack of IS. It's a bit too long for most portraiture in my opinion. As for the lack of IS...it's an f/2 aperture, but if you buy a lens to shoot handheld at 1/15 of a second, you're out of luck there. I have other lenses for that, though.
To each their own...in my opinion the Canon 100L is a poor value...the Tamron and Sigma stabilized macros are just as good for 20 to 30% less money. The Sigma is now well under $600!