I agree with a lot of everything thats been said above this post so i won't just repeat it. i would add that in my opinion it creates interest in the mix. you can fold the dancefloor by dropping different tracks with different emotional feelings, and the magic of getting into and being part of a set that you can feel the energy and the vibe from your audience... well it just electrifies me and is probabliy why i am a dj/drummer