William -- Canon T2i & EF 200mm f1.8L @ f1.8, ISO 6400. Click for larger. This picture looks horrible in color, but is actually not bad when converted to B&W. To me, the T2i/7D ISO 6400 is only usable in Black & White.For most people, they need want fast lenses so that they can shoot at low light or have high enough shutter speed to stop the action; some will buy them because they are less common. A small percentage would buy high speed lenses for the super thin depth of field affect. Of course, others will buy them because they could. Personally, I like fast lenses for the first three reason I listed above, but mostly so that I can take blurry-free pictures in low light.
With today's new crop of sensors that are so good in high ISO performance, we can take relatively noise free pictures at ISO 3200 or higher. Your slow f2.8 lenses are no longer the limiting factor, if you can accept the slightly noisy pictures. So, if you had a very capable camera that can take clean pictures at high ISO, given a chance, would you still buy that f1.2/f1.4 lens that you have always lusted after? The price of a Canon EF 24mm f1.4L II and the EF 24mm f2.8 is $1849 vs. $419 CAD (approximate, at Henrys.com as of today). The difference is two stops faster for the f1.4 version over the f2.8, but at about four times the cost. Well, yes, there is more to an expensive lens than just letting more light through. The built quality and materials used are better, but under normal use, I doubt there is much difference in logivity.
There are legitimate reason for fast lenses, especially if you are into actions/sport photography. I know that my 200mm f1.8L is often not fast enough for indoor use, so for this kind of uses, we need all the speed we can get.
But, most of the time, for me at least, I often buy stuff irrationally. For example, the 50mm f1.2 vs 50mm f1.4 Canon lenses. It's a 1/3 stop faster for the f1.2, but many times the price over the f1.4. When I bought the lens, I fully understood that under normal use, they practically have zero difference shooting from f2 on. Yet, I bought the more expensive version. What justified my decision? My reason was really simple. I wanted something less common than most others. This also explains why I have "made" so many of my own lenses from projection and enlarging lenses, because majority of people would not have these kind of lenses, and therefore, pictures that look different from these kind of lenses.
Of course, all of these talk is nil if money is the primary factor. I would not be able to do it again today, because my personal disposable income situation has changed. If I wanted the 50mm lens today, I would be buying the EF 50mm f1.4, not the 50mm f1.2L. So, I think another factor in buying faster lenses is because we could afford it.
From my point of view, for most people, the faster lenses are no longer a neccesity to take blurry-free pictures, and they would be happy with a slower lens. For lens junkies like me, we want fast lenses. In fact, the brighter the better!