give me a good reason why they prefer those extra channels
In the DJ segment of the mixer market you got scratch/battle mixers, dance mixers and clubmixers.
For just plain back to back mixing or scratching 2 channels is perfect.
A third channel becomes necessary when you're into fast/battle mixing and/or live remixing.
In clubs you even want more channels for greater flexibility, eq. DJ's who work with vinyl, CD's and DVD's and for external effects.