can you outline for us the pros and cons as seen through your eyes of using and not using the leader lines?
I'll answer that one, if okay ?
When you are saying using leader lines, I assume you mean long
leader lines, at least half a kite span - so you have the safety leash attached to the end of the leader line.
And when NOT using leader lines, we are still talking about 1 meter or so, at the bar.
You must have this very short leader line for a few reasons:
1. Your fingers/hand could be seriously injured or ripped off, if the flying lines where attached directly at the bar - because you might get caught.
2. You are able to use flying lines of equal length.
3. You wont cut your flying lines with the edge or fins of your board, when relaunching.
Using long leader lines :
Advantages: None, really.
Except if you are using bridles for reverse relaunch - then you might want leader lines of at least 2 meters length. But this can be done with a normal front line ring-ring safety leash too ("De Facto" standard today).
1. When the lines are twisted after a jump, your safety release might not work at all, because of the knots and the leash beeing squeezed into a "lock".
2. Even after a single rotation, you can not control the kite very well, because the knots will lock up.
Long leader lines is a thing of the past, but of course we see a lot of those as they are standard on older kites/bars.
And for beginners who drops the kite often, but never do rotations - it can work OK.
But otherwise, there are only disadvantages with the long leader lines
Regarding line length - I use 27 to 28 meter from bar to kite, a very good overall performing length IMO
Shorter will decrease the lower wind range and float in jumps, and longer will decrease the "pop" and height in jumps (for my taste at least...).