I.S. cannot help stop blur from subject movement if you are forced to use slower shutter speeds indoors. I.S. only helps reduce blur from camera shake.
Now, sometimes subject movement blur can be used to good effect in images.
And both camera blur and subject blur in an image can be pretty bad.
I.S. is very useful, even for indoor sports. Just don't expect it to stop subject motion. If you want to freeze the action all or some of the time, you still need a larger aperture lens and/or a camera that allows you to use higher ISOs, both of which can allow you to use faster shutter speeds in lower light situations.
Sorry, but I have to disagree. For example, 1/400 is a huge difference from 1/100 when trying to stop subject movement. So f4 > f2 can be the difference between being able to get the shot or not.
The choice between primes vs lenses most often comes down to whether you can move around and get the most out of a prime, or if you have limited movement around the event and need to quickly zoom from one part of the playing field to another with your lens. There can be times when one or the other is possible, so it's nice to have both types of lenses available. The ideal is the largest aperture lens you can get, with I.S. Second choice would be the largest possible aperture lens without I.S. At some point, though, a large aperture also can start to make depth of field too shallow.
