#4
Because normal scales use different intervals to make it sound musical, with different scales associated with different moods. Try using the chromatic scale to write a lead riff. The only one listening will be Ben Stein.
#5
no.

If you do that you will end up sounding terrible. Every key has specific notes in it, so stick to the scales
Paul Gilbert
Steve Vai
Joe Satriani
John Petrucci
#6
The point of scales is to find notes which work well together in a melodic manner (or not, depending on your mood).

Having "all of the notes" in a scale, defies the reasoning behind scales all together.
I bet you five bucks that I play guitar.