Auto ISO is a convenient feature AND a bad habit :-).
From the following ramble:
If you have a superb high ISO performing cameras (D700 up) auto ISO may be marvellous
For studio and fixed use or when you have time to think it is of very little value.
For walk around situations with mere mortal cameras, upper ISO setting is liable to be noise constrained in a given setting and lower ISO setting is liable to be blur or subject motion constrained. Chances are that setting a wide ISO range will hurt you more than help.
So, as long as your camera allows rapid ISO changing and as long as doing it is etched into your brain so it can be done without thought, auto-ISO does not really offer you a vast amount and can reduce rather than improve your end results.
I have a Sony A77 with max and min of the range settable on Auto ISO.
For gneral purpose use I use Aperture priority mode, but may use anything else that suits if required.
If I use auto-ISo at all I usually set it over a rather tight range - say 400-800, MAYBE 400-1600.
The most major problem is probably that the camera does not know what your intentions are, and ISO setting can affect not only noise levels but also eg motion performance. I recently took a photo from a moving car with the camera set to auto ISO 400-1600. It was a spur of the moment shoot-it-or-lose it occasion with no time to alter settings. The camera decided it was a nice bright day so 400 ISO would be just fine. I lost 2 stops of shutter speed on a shot where blur rather than noise was important and the result suffered. If I had been on foot outside then it would have been a fine choice. But by the same token, if 400 ISO was OK for wandering around I may have used it and if a dark doorway or similar demanded it I could have made the choice between extra care and bracing or ISO changing.
A question set worth asking is, under what conditions would a really wide range of ISO settings be acceptable?, and if they would not be, what advantage do I gain from a say 1 or 2 stop range?
If you are using a flash, or taking a group or semi formal photo (wedding, 21st, family group photo) then you would usually have a reasonable exact idea of what you want to achieve. Not only ISO but aperture, white balance setting , any dynamic contrast adjustment options and effects or filters or, ... . With perhaps shutter speed being the free variable which allows the others to be preset. If you do all that and then shutter speed is too low you go back and change something. Auto ISO is unlikely to help in such a case. ie if 50 ISO is preferred then you are unlikely to be happy with 100. If 800 is acceptable would 200 cause unexpected problems with subject motion or camera shake or ... .
I do a fair amount of what loosely fits into the "street photography" category - mainly for fun but with some paid jobs (also for fun of course :-) ). . Party, sports casual, children at play, street scenes, walking through a fair or festival, literally people in the street (Ladies Market in Mongkok, HK is hard to beat for variety and interest, but it may also be my local mall ), ... . With the A77, up to 1600 ISO gives an acceptably good result for me in most such situations for spontaneous spur of the moment shots. I almost invariably use aperture priority. A child running needs larger aperture to boost shutter speed, plus good care in focusing accordingly. People walking or stationary allow f8+ in most cases. If a group of people pose or take time to let me interact I'll usually drop ISO to 400 or so allowing smaller aperture, and deeper depth of focus. A dark alley way may tempt me to ISO 3200 - but I'm more likely to try bracing against a doorway and Ninja breathing.
THE POINT of all the preceding, is that in few of those case would auto ISO be my friend. The running child does not want the camera to choose 400 ISO. The group shot may as well have the acceptable 400 ISO quality or lower. If i decide to use flash I may drop to ISO 200 or 100 deep-ending. If I drop off the 18-250mm walkabout lens that I usually fit and fit the 50mm f1.8 or 500 mm f8 mirror that I carry almost everywhere then I also have a very very good idea of the ISO that will match the reason that made me change lenses.
SO for me auto-ISO is of some use, but when I am "out and about" it is liable to get in the way of what I want to do, because what I want to do is very variable. If you have a camera with superb low ISO performance (say approaching D700 capability on up) then you may be able to afford a wide ISO range, But with more mortal cameras you tend to be hemmed in at one end by noise and at the other end by potential blue. Auto-ISO is still useful, but I can easily live without it.
One last factor is ease of ISO change with the camera being used.
With the A77 the ISO button is just rear of the shutter button - slide index finger back and press. Then either roll the front adjuster which is under your right middle finger or (milliseconds slower but what I actually do) flip index finger onto front adjuster and jump ISO up in multiples of 2, then finger back again to shutter OR roll rear control which is under your left thumb to step ISO in EV/3 or EV/2 steps. Very fast. If your camera takes many seconds to set the ISO setting the options may change.