I think its because harness lines are nice and stiff and hold their shape making hooking in and out easier. Plus they dont twist as easily and hook you up when you catapult. Thats my theory anyway.
I use the sailworks 'quick-tune' harness lines that are just rope and a cleat, with a bit of PVC tube over the top of the rope.
Without the PVC over the top of the rope it doesn't last long and it's not too much fun when it does break... With the PVC tubing over the rope it doesn't swing too much, but I can imagine that without it, it would be a pain to try to hook in. The stiffness of most harness lines help a lot when you go to hook in.
Way way back in the mid to late eighties (1986) prior to having a twenty year hiatus from windsurfing, we used to make our own harness lines with rope, but as stated the rope would easily break. But then we discovered the high pressure air hose lines like those you see on Semi-rigs and in factories (it's coiled when not extended) and cut about a three quarter circle of the coil and feed over rope with knots either end to keep in place. Never broke the first lines I made and I think the boom with lines intact is still underhouse somewhere. The coil acted in much the same way as harness lines do now although sometimes the harness line would wrap around boom (although never twist).
[Now there's 30 seconds of your life you're never going to get back reading this really boring story]
I just got a new set of Dakine Comp Adj harness lines.
After removing from packet - I swore, WTF are these with this massive SS cleat?
Putting them on the boom - I swore a little more, 1/2 hour job very tight!
Used them for the first time on the weekend. What a dream 
they are so easy to adjust while under load for lenght in and out. No need to remove all load from lines to adjust as will all other sets i have used.
The grip onto the boom was super with no movement and the locking cleat was very sturdy with no slip all.
Very happy with them indeed well worth it.
Hey Tim,
Just watch the webbing on the adjustable part -- both sides of mine broke after a couple of weeks. The stitching was done really close to the edge of the webbing, and it just pulled out.
I replaced that loop with a homemade webbing larks head, worked a treat and lasted until the rope itself broke.
You're right though, the SS V-jam cleat is the shiz.
