Two words here are professional standards. You wouldn't want to shoot a wedding without a 2.8 lens, in a church. With a fast lens you do not have to use flash (or can use it just to spicy up the photographs, not as a sole means of illumination). Of course many slower lenses are as good optically, some even mechanically (although an exception). It's always easier to design a slower lens to be good. But market thinking creeps here. Pros want 2.8 glass, so most other glass is not built for pros but for "amateurs". Thus expect lower build quality and (sometimes) worse performance.
I shot with both 2.8 and 3.5-4.5 short zoom (20-35) and the faster lens made a whole lot of difference. No matter where I shoot, I mostly used the lens wide open inside. So the speed paid for itself. That to say, I liked Pentax for providing quality alternatives, for example for hiking - 2.8 zoom is to heavy to lug around when you hike, and Pentax always had high quality lenses for those people. 24-35 f/3.5, FA 20-35 f/4, and similar. To the mountains, I would not take a 2.8 zoom for issues of weight. For the occassions I need fast glass there, I would take a small fast prime. Frantisek
