Because it's great fun.
Skew, to the best of my knowledge (one of my lecturers at uni), the nausea is caused by a form of travel sickness: if you are standing, facing the direction of spin, and you rotate your head 90 degrees to the left, your eyes tell the brain that the body is stationary, but because one ear is accelerating into the direction of travel, and the other accelerating away, the perception is that the head is tumbling forwards, as this is not backed up by the eyes, the nausea results.
Cold, that is an idea that's been trialed, NASA looked at it on some of the Gemini missions (11?), and were able to generate a small measurable (but not felt) acceleration around the ATV using a long tether. It is probably the most practical method for a small habitat at the moment, but it would be very difficult to dock any spacecraft to the module, due to the circular flightpath, unless you had a docking tube running to the center of rotation.
one proposed method is having a centrifuge area where the astronauts sleep, as they'd be immobilized they could be exposed to much greater rotation, hence the module could be much smaller. as a result it could be part of a ship (although from memory the destroyer from B5 didn't have anything big enough to be realistic)