You will find that in many cases, the faster lenses, are more expensive, and hence have better overall quality IQ and glass as they appeal to a more demanding professional audience.
True, that as ridiculously high ISO's are usable in the advanced DSLR world, the necessity of a fast lens for decent images in low light situations - as we used to have in the film days and older digital days - is less.
Coupled with the new IS feature found in the lenses, allowing slower TV exposures, large AV's are not as important as far as exposure.
However - large AV's also have a creative affect on images as they allow much more shallow DOF - an affect you will often see in pro photography. This unique feature is not possible with slower lenses.