I'm hesitant to jump into this speedlite conversation, but what the heck.
Seems like you all are making this more complicated than it needs to be. In camera's manual mode, set the shutter speed and aperture for whatever you want (within the sync speed limits of the camera and strobe). With ETTL, the strobe will generate the right amount of light to expose the subject.
If you want a lot of ambient light, use a slower shutter speed, higher ISO and/or bigger aperture. If you want the speedlites to dominate, increase the shutter speed, lower the ISO and/or stop the lens down. Speedlite adjusts to match the exposure you picked. If the shot needs more or less light from the strobe, adjust the flash exposure compensation.
Since much of this thread has been about letting the technology do the grunt work for you, why not do it with ETTL. What am I missing here?
Changing the ISO or the Aperture affects both the speedlite and ambient for more or less light. Hence your really stuck in the same position but your speedlite will have to work harder instead of using shutter speed to control the ambient.
I concur on using ETTL for the speedlite control, but keep the camera in M mode. We're not talking Manual flash settings, we're discussing Manual camera settings VS Aperture variable settings on the camera. If the speedlite starts to get hot from all the pre-flashes, I'll just switch to full manual on the speedlite.
I'm not saying that Av + Flash is completely useless. It works great if the light isn't changing, which is a kinda strange irony. Overall, 90% of the time I use a flash, It needs to be in M Mode.
I'm really trying to learn everything in manual...but like others have said, a time and place and tool for everything, no?
With flash, I kinda go with what I heard Syl Arena say, that he only uses ETTL when the distance between the lens and the subject are moving....which makes sense if you're out shooting runners in a track meet. If in the studio on tripods, then manual is likely the way to go in most instances so you have more control and can generate more precise custom looks, the way YOU want them to look, not the computer in the camera.
As for camera, I'm somewhat like minded there...I try to shoot manual exclusively when at home shooting things on tripod, like still lifes, posed models, or landscapes...
Even run and gun, if I can, I try manual, I set ISO, and aperture and when shooting,I spin my shutter speed till I get the exposure meter about right...
If things move too fast past that to keep up with, then I switch to AV mode....etc.
But again, I'm a total noob, my 5D3 is my first DSLR and I'm trying to learn everything it *can* do, and keeping in manual most of the time, is the best way to figure things out IMHO.
Immersion learning so to speak....