It's all in your methodology and what you're willing to compromise with.
if you're picking the shutter and aperture, it's for a specific reason and then you're saying: okay camera, use your meter and exposure based on my settings the best you can using only the ISO. Any changes is light will only be handled by changes in the ISO. This could easily ramp up to ISO 6,400 if it needed just 3 more stops of light at a base of 800.
But what happens if you're trying to shoot at say 1/250 on a bright day, and f/2.8, and a base of ISO 100...The camera is wont be able to auto-adjust down if the meter is saying youre over exposing shots. And even if you decided to look at a histogram between shots, and then dial in any EV really fast, nothing would happen, you'd have to dial up the shutter speed or aperture on your own in order to remove more light.
at least if you were shooting just aperture priority it could be limited to 1/250 at the lowest shutter speed, and then it could ramp all the way up to 1/8000 if it had do while still keeping ISO 100 as a base first and foremost, only then sacrificing ISO in order to achieve the exposure.
To me, every shot is different. Last night I used auto-iso, shutter priority, and spot metering in order to shoot the hawk in my backyard.
I went with spot metering because I knew there'd be times where I might be shooting towards the sun, or with trees in the background that could influence the overall exposure.
I went with auto-iso for similar reasons. I wanted to make sure that the hawk stayed exposed regardless of the on-the-fly changing lighting conditions. I was using a lens with a 4.5-5.6, so if I needed more light, there wasn't going to be much on setting sun as far as aperture stops anyways.
I went with shutter priority because it was most important that I stop motion/shake first and foremost.
...a few hours before that I was using manual mode and manual iso taking flash portraits.
...a week before that I was using aperture mode with auto-iso and taking indoor flash portraits.