Fast lenses, depth of field, and Photoshop CS6

[Alan]

Depth-of-field is not the only advantage of a fast lens. A lens with a maximum aperture of f/2.8 lets in twice as much light than a lens with a maximum aperture of f/4. That could mean the difference between a motion bluring 1/60 sec or a crisp 1/120 sec at the same acceptable ISO.

True. But, those occasions are much more limited, and probably more often than not, it's the depth of field that will be preferred (portraiture is a good example).

Plus, with newer, low noise, high ISO cameras (plus, greatly improved noise reduction software), the need for the heavier, fast lens could wane.

Certainly, the cost could be a significant deterring factor.