I think the reason to use one is low light, and low light means a pucity of photons resulting in less clarity. That gives the lenses a lot latitude. I still have figured out how to adjust the shutter speed, or even the real full name of the camera! (El Cheapo Pen). I will be looking for an f1.4, and maybe a telephoto, such as the 75mm you mention. I think it is Pentax too. I kind of like the round pictures.
BTW, during the SLR renaissance, 1.7 or 1.8 or even 1.4 was the norm for a standard lens, yet the digital people think we should be happy with a 3.5 zoom -- what is up with that? Renaissance over? (hmmm....)