Over the years, there has been much debate over the use of the up-faders and the use of the cross-fader. Many DJs when they are learning, use the cross-fader to mix as that is the easiest way to visualise the horizontal transition from a track one side of the mixer to another. When I started DJing, the volume up-faders remained at max for the whole set and the mixing was done entirely on the cross-fader. This worked all right but the revelation that mixing is easier on the up-faders improved my sets a lot.
Let's first of all look at the difference between cross-faders and up-faders. Each up-fader corresponds with the volume of one channel AKA one song. It offers resistance when pushed so is quite accurate for minor adjustments.
The cross-fader on the other hand controls two channels, one side is maximum volume for track A, with track B silent, the other side is maximum volume for track B, with track A silent. Cross-faders are also generally looser, good quality ones are exceptionally loose and this is so you can move them with minimal effort. This is usefull for scratch DJs who want to cut songs in.
The cross fader can be set with a linear curve where as you go from one side to the other the tracks respectively lower and raise their volumes in a linear fashion (far right on below image).
The image represents the volumes of the tracks along the x-axis of the cross-fader. The left image demonstrates that as soon as the cross fader moves slightly to the right, the volume (slightly lighter grey) of the right hand side track increases steeply.
This is essential for scratch DJ's who want to play snippets of the scratch track at full volume over the main beat track. for this, they don't want to have to move the cross-fader to the exact middle point, they just want to move it and have the track kick in full volume, and then when the scratch is done, they want it to cut out immediately by moving the cross-fader back to the side.
This video demonstrates the technique used by scratch DJs and shows why its important for the cross-fader to open fully instead of gradually.
So, back to mixing. People who don't scratch say they may as well just have a linear curve on their cross-faders as this scratch thing isn't necessary. I disagree. I have my cross-fader set up like a scratch DJ's, even though I don't scratch and there is a good reason for having it like that.
When I perform, I like to have complete control. I don't just switch from one track to the next, I like doing long mixes that require tight EQ's and careful volume control. The resistance of the up-faders allow me to do that better.
Secondly, it means I can play about with the cross-fader as I'm mixing. The way it works is if the up-fader is set to halfway, that is the maximum volume the cross-fader will play that track. This means I can cut in beats, vocals or melodies at volumes appropriate to the mix.
If you do decide to mix with your up-faders, make sure that the cross-fader curve isn't linear, it means that the cross-fader can be anywhere in the middle and the volumes are the same.
One of my favourite techniques is having a nice dancy house song on track A and then track B is dedicated to a song with a good hi-hat. I take the bass and mid EQ of track B and only open the cross-fader to the hi-hat, this adds energy to track A while introducing track B's hi-hat.
As the mix progresses, I'll leave the cross-fader open and mix in the mids and bass appropriately (see here for more in depth EQ'ing a mix).
Essentially, by mixing with the up-faders, you allow yourself an extra control for expression, complexity and style that you can add to your set. The cross-fader is designed for usage like that described above and so there is no point wasting such an expensive component built into your gear.