I constantly see people talking about 2.8 like it's a godsend. In real world shooting, if you can't get an image with f4, how likely is it that it will be a great shot at f2.8?
'Way back in the early 70s--when f/2.8 lenses were actually kind of rare in most focal lengths--I discovered something interesting. At the lowest light levels humans tend to be commonly comfortable with (the lowest level before they begin turning on lights), with ASA 400 film (the fastest film before grain became too obtrusive), f/2.8 usually gave me a shutter speed of 1/60 in a great many more situations than f/4 would have.
It was an odd--but common--kind of thing. An f/2.8 lens was significantly more likely to give me a shutter speed that was easier to hand-hold and stop human movement than f/4. Sure, an even faster lens would provide a higher shutter speed, but besides the fact that they were rare above and below 50mm, faster lenses introduced more depth of field limitations.
The f/2.8 lens of any focal length is a sweet spot about as fast as you can get without depth of field issues. It just tends to work out that way.