I can't speak for everyone, but I own multiple teleconverters and frankly I kind of hate using them. In the field or at an event, I often don't bother with changing it out because of the environment, the amount of time for the switch, or the risk of getting gunk in my body during the change. The 135L takes a 1.4x quite well, and I do use that combination, but let's be realistic; it is only 189mm, and an unstabilized 189mm at that.
The 70-200L II takes teles very well well, too, but even on it with a 2x there is quite a degrading of image quality, and it is a 400mm f/5.6 by that point.
If you want reach, the Tamron is the best bargain option on the market. I'll certainly be adding one to my kit. And for those saying it is only good up to 400mm - they haven't used it. I thought I was taking this wide open, but it is actually f/7.1 (stopped down 1/3rd a stop). But it is 600mm, handheld, and ISO 2000. How exactly is this not usable?