I rarely use auto ISO because unlike changes in aperture or shutter speed, changes in ISO influence the quality of the image recorded on the sensor. If you increase ISO, you decrease the signal to noise ratio and dynamic range. I just want that compromise to be a deliberate decision on my part when it is necessary.
I don't want to blow white plumage on a bird because I forgot I had auto ISO set. My left thumb is now pretty well trained to find the ISO button while my right thumb spins the command dial - and I know what I have just done to the image.
I imagine there are a host of circumstances where there is a known acceptable range of ISO settings that will have an insignificant impact on quality while providing shooting flexibility - but it's just not a setting I'm comfortable leaving on the camera.