Lack of IS really hurts the 70-200? I think that's something you've cooked up in your head...the 135/2 also lacks IS...the versatility of the 70-200 is its strength...the moderate size and moderate weight of the 135/2 is its strength. Last year, I shot my 70-200 L-IS USM without the IS on for a couple of weeks..I was surprised how well I did with it OFF...
I dunno...the 135/2 L has this "legendary" status, but it's no better than Nikon's 20+ year old 135/2 AF-D Defocus Control lens...in fact I own both the Canon and Nikon 135/2's and one summer I shot LOADS of images with both lenses, swapping one in for the other ALL DAY LONG one summer day, using the Nikon on the Canon 5D body, and the Canon on the 5D...at the end of the day, I converted all the images to large, high-quality JPEGS in a batch process...I couldn't remember which shots were made with which lens...they looked virtually identical.
I am not sure where this "legendary" 135/2-L mystique comes from, I really am mystified by it...I prefer the focal length flexibility of the 70-200 lenses, especially on crop-body cameras, where a 135mm prime is very limited in where it can be deployed. Most 135mm f/2.8 primes of quality manufacture are quite good lenses as well...and are HUGELY smaller and less-obnoxious. Canon's small 135 f/2.8 SOft FOcus is a nice, small,compact,light lens. I have one I picked up cheap...people simply do not like the idea of that lens--it is a terrible lens for re-sale, so buy it used,like I did.
The way I look at it, the idea is to keep building one's lens collection, not trading off lenses, or losing money when selling off lenses. if you have a FF body, the 135/2 makes at least some sense. If you have a 1.6x camera, it makes very little sense, except for certain,specific types of scenarios.