1) If you are interested in a sculpting tool without the need for a modelled base mesh at the start, have a look at 3D Coat, especially the Voxel sculpting part.
Basically the tool works in a way where you can start with primitive objects (like cubes, spheres, but also imported meshes) that get converted to voxels (3D pixels), and then you can work on that without worrying about the topology of the model (because you are working on the voxel model underneath, and that model gets converted to a polygon mesh for diplaying in the viewport automatically).
Voxel Sculpting has its own set of limitations and shortcomings, so its not the silver bullet some people are looking for, though.
Something similar can be achieved, to my knowledge, with ZSpheres in ZBrush (minus the part of not worrying about topology while you sculpt)...
1a) About being faster sculpting than modelling... that depends now on what you do... roughing in a mechanical model in 3D Coat Voxels vs a poly modeller is most probably not faster, maybe even slower. And depending on how much smooth curvy surfaces you have, both will be blown out of the water by a good NURBS / CAD tool... that is why its good to learn to use NURBS tools.
If you are adding highfrequency details to a model, or try to sculpt the musculature of an organic model over its basic structure, you will be hard pressed to do that in a poly modeller in any efficient manner.
That is why both kind of tools are usually used for different phases of creating a model. Poly modelling and NURBS / CAD modelling to create the base model, Sculpting to add the details or refine the basic organic sculpt (that might be just a skeleton at this point in time).
2) About the hair: traditionally hair was added at the end of character modelling, when the finished sculpt was taken back into a poly modeller and hair was added as simple flat meshes that represented a single strand of hair... add 50 of these, scatter them around the head and angle them correctly, assign a nice Hair texture with an alpha channel, and you get nice looking hair that is actually not that hard to create. AND will not look like a plastic wig like what you can achieve with sculpting.
Now, with some of the new systems being developed by Nvidia and AMD, that might change (TressFX Hair system for example)... no idea what the workflow is there.
3) If you are talking about animated movies: actually, there high poly models are used. That is also why it needs to be rendered offline by entiere farms off very powerful computers. For your usual PC, that models would even take long to render if you just rendered out a single frame... at interactive frame rates, that is not possible yet with current hardware. So for games, high poly models are too demanding.
And thanks to bump mapping and other graphical tricks, its also not really needed. There are cheaper ways of making a game model look more detailed than it is, and while you can still spot its low polygon nature sometimes during the game, you will mostly not notice that at all.