Really good answers aren't they?
I know people moan and groan about the beatmatching thing
but he's right, essentially a DJ has to be able to do his thing in real time
and make those choices of tunes, and no, you can't program a computer to do it for you
there's just too many variables, you have to be there selecting it yourself.
the old dub dj's never used to mix , in fact they normally only had one deck :
play the vocal side
play a silly noise with a wozzle ( sound fx ) box whilst flipping it over
play the dub on the b side ( mc chats over the top at this point )
REWIND SELECTA !!!!!