Do Fast Lenses Matter Anymore?

This is precisely my thought on the matter. It's not an either or proposition. Improved sensors let you get away with using a slower lens in many cases, but they don't effect the DoF in a way that would replace the faster lens.

Plus, I'm often times shooting in a park that's in a narrow ravine and I'm usually shooting relatively wide open to get enough light. The light that my F2.8L gives me over the F4L is significant. Not all the time, but it does mean that I can sometimes get away with going to the next ISO.

Plus, you buy the lens once and chances are good that it's going to last a goodly number of years anways.