The AC was pushing the ETTR (Expose To The Right... of a histogram) philosophy, which says that you should expose as much as you can but without clipping. That's fine for still photography but the trouble with doing that for a narrative scene is that every shot in a scene could end up with a different exposure simply based on whatever the brightest thing in the frame was, so the subject itself from shot to shot could be exposed all over the place and then have to be timed in post to match again, so dailies that don't get color-corrected would look all over the map in exposure. Plus your noise level would vary, the angles where you could expose the subject more would be cleaner than the shots where you had to underexpose the subject to hold some bright element in the shot. It could end up being like, in terms of base noise, intercutting one close-up shot at 200 ISO with another close-up shot at 3200 ISO.
In narrative filmmaking, continuity of look across all the set-ups in one scene is more important than exposing each individual shot to get the least noise possible. If you were shooting on film, you might pick 500 ISO stock because that's the best ISO for your light sources and stop you want to shoot at, but you could light closer shots easily to enough level to switch to a lower ISO stock, but you don't because it's important that the scene have a consistent grain level.
Also, if you merely exposed for maximum signal level, then you'd have to fix so much of the timing in post -- imagine an actor who stands next to a bright window, then crosses through a dim area and is a couple of stops down, and then end up in a hot overhead lamp, and you shoot a wide shot where all of these changes happen in the frame, and then you cover the actor for each position in the room separately. Are you really going to expose each section with no regard to how it looked in the master? When he's in the hot light in his close-up, the exposure brings him down to normal? And when he stands in the dim light, the exposure brings him up to normal? And then have to correct those shots in post to match how he looked in the master shot?
If you have to tweak the exposure due to the limitations of digital, to hold some bit of detail in the shadows or highlights, you may have to do it, but in general, you pick an ISO level that gives you the noise that you want for the scene and is practical, and then you expose within that setting for the look you want, mood-wise. You can be a bit conservative if you want, not push a dark moment as dim as you want to achieve later in color-correction, just to be on the safe side, but in general, you should expose as if you will be getting a one-light print, i.e. the straight dailies should have the correct exposure for the look you want without needing shot by shot color-correction. You cheat when you have to, but you generally expose consistently for the look.