I was looking at my lenses recently, thinking what do I need / want? I realized I don't have an optimal low light telephoto lens. The 70-300L has IS but is rather slow - the 135L is 2.0 but no IS.
The current Canon 70-200s are all a bit short compared to the 70-300L, and I can't really justify owning two zooms. So what I really want is a fast prime with IS. But the only ones Canon makes are the expensive pro lenses (200 2.0, 300 2.8, etc.) and the 100mm macro (still a bit slow at 2.8). For whatever reason Canon has not come out with a reasonably fast 85mm, 100mm, or 135mm lens with IS. Why?