1 you don't, its just that if you want to be able to shoot half decent video, a point and shoot won't cut it. A DSLR can do both (if you invest in the right lenses and stuff).
2 not sure what the exact difference is in image quality and all (obviously a dslr is more depth of field and you have more control over all of the settings) between them. A dslr is only worth it if you invest like $1000+ in good lenses and tripods and stuff.
3 For mac, you can get either fcp or adobe cs6 (or vegas I think, i dont use vegas so idk) and on windows you can only use adobe or vegas, not fcp. I believe that fcp is the only one that only works on one OS. Also, obviously a more powerful computer is going to be faster and work better for editing
"take deoxyribonucleic acid for example. Sound's impressive right? Ever see what happens when you put something in acid? It dissolves. If our bodies were all full of that acid, we'd all dissolve. So much for the theory of evolution."
"When there are more people in your organization who believe Obama is a Muslim than believe in the theory of evolution, there is a problem."