Having just finished searching for the answer to this question on the web and within forum. (doesn't mean its not there, I just did not find it)
Does anyone know why I would not want to just turn on the ISO expansion and leave it on?
With my 10D I never liked anything above ISO 400 and would only go to ISO 800 on rare occasions. ISO 1600+ was useless to me in my opinion so I did not bother with ISO expansion on my 10D, on the flip side with my 1DsMkII the ISO 3200 looks a bit better then ISO 800 on my 10D, so there will be times when I may use it.
Since Canon felt the need to put it in a optional category, I would presume there was a reason for this.
Any one know the reason?