I like fast lenses because they give me options to shoot at low light when using slower lenses is not possible to get the pictures I want. Sure, you can crank up the ISO to compensate, but as the light dims, there is only so much headroom you can get from ISO. We all know that as the ISO increases, the image quality decreases. Fast lenses allows me to shoot at lower ISO and still maintain sharpness with fast enough shutter speed.
Fast lenses are also made better, especially with modern lenses. This was not really an issue in the past, when lenses were all made with longevity in mind. If you compare an FL 50mm f1.8 and the FL 50mm f1.4, you will notice that they were practically made identically, except the optics, but you can already tell there is a difference between a New FD 50mm f1.8 and the New FD 50mm f1.2.
Freedom of depth of field control is another benefit for fast lenses. For many people, this is the major reason to buy fast lenses, but like everything else, there are situations where thin depth of field is not appropriate, and there are situations where creative use of thin depth of field can make your pictures stand out from the norm.
If you read my previous posts, you will see many pictures with very thin depth of field. I like to isolate my main subject from background. Looking at the pictures with very thin depth of field immediately tells you where the main subject of interest is. The most interesting bokeh is usually at the widest aperture, and unfortunately, I am an incurable bokeh addict. That's why I have so many pictures with thin depth of field.
The question comes down to this: does it matter how you use your lens? Absolutely not! A lens is a creative tool. If your creative vision is best expressed at f32 with an f1.4 lens, who's to say that's wrong? The softness from diffraction limit combined with huge depth of field may create interesting pictures. Of course you would argue why not just buy a cheap slow lens, but what if you want to shoot at f1.4 the next minute? See, fast lenses give you a choice that a slow lens can't.
I am addicted to fast lenses. It really is an expensive and unhealthy addiction, but I enjoy it nonetheless. Some like fast cars, and I like fast lenses, and I use them anyway I like.
Hands -- NEX-5 & JML 50mm f0.95 @ f0.95. Click to see larger.