Cons include no 70mm, 85mm, 105mm, 150mm and 200mm prime focal lengths all in one lens. I would hazard a guess that the AF is also better on the 70-200mm.
Well I have these things called legs and use manual zoom
135L is my longest reaching lens and I've even been to Richmond Park and stalked deer with it. It's fine!
And a 70-200 does not give you a prime focal length, it gives you a much greater range but you know as well as I do that a prime will always be sharper than a zoom and the 135 is faster as well.
See, I'd hazard that the 135 has better focusing. Its stupidly quick. If there is a difference between the two, I doubt anything is in it.