I use Serato because of Scratch Live... whenever I messed around on vinyls over my friend's house he would use Serato for loading tracks onto his dubplates. I have nothing wrong Traktor, to be fair you can play without software easily enough, it just seemed like the obvious choice at the time as I had previous experience with Serato (plus intro DJ came with my midi).
The MIDI frustrates me because I find having 4 physical decks in front of me much easier to control. When I'm switching track every 30/45 seconds, I personally find it's easy to forget to switch decks. It means that I sometimes accidentally stop the track playing instead of playing the next one ha.