I've had the 100/2.8 Macro non-IS for years and use it on both crop and FF cameras. I like it a lot. Don't really need IS on a macro lens, since I'm usually using it with some support anyway, and it's not good for much more than a stop of assistance at higher magnifications.
I didn't get the 135/2 until I bought my 5DII.... then I got it at the same time because I knew I'd want it.
Someone posted here a while back, put it into better words than me, that the macro lens is wonderful, sharp as a tack (either of the 100mm, take your pick)... but is sort of "scientific" in it's image rendition. It's never been my favorite for portraits and that's probably why. The same poster described the 135mm as a more "artistic/painterly" lens. I have to agree and love it for portraits and some other applications.
In other words, the 100mm macro(s) and 135mm have very different image rendition and character. So, I use both and plan to keep on doing so.
Yes, it would be nice to have IS on the 135/2.... but it also would add $300 or $400 to the cost, and I don't really have trouble handholding it. I have 70-200/2.8 IS, for those times i need IS, anyway.