Um.. I don't think that's quite correct. Have you tried using an external light meter?
Let's say you use a point and shoot camera with a crop factor of 6 (I think that's about 1/2.5"). Any way, to get a equivalent field of view of a 35mm normal lens, the camera uses a 6mm lens.
Now, let's imagine taking a picture of an object 3 meters away at f2 with a Canon 5DMarkIII using those two lenses: a 35 mm normal, and a 6mm fisheye....
According to Depth of Field Master, the respective depths of field
6 mm: 0.5 m-- Infinity
35mm: 2.62 m -- 3.51 m
Fisheye lenses are notoriously easy to focus...
Put that 6mm lens on your P&S camera, and sure, you'll have the same field of view as your full-rame camera, but nearly everything will be in focus, including any distracting background. A large sensor lets you use depth of field for artistic effects.
Besides, f/1.4 lets in twice as much light as f/2.0, and four times as much as f/2.8.
ISO is (at least theoretically) calibrated so that two cameras can be set to the same aperture, shutter speed, and "film" speed, and produce images that are identically exposed. However, that does not mean that both images will be absolutely identical-- a Nikon Df set to ISO 6400 will produce a image that is considerably less grainy than a Nikon D3100 set to Hi1.
That's why the Df is so intriguing--it can be used in exceptionally low light, yet still produce a clean, detailed image.