I like the idea of a short leash and I'll probably go that way in the future.
I have never really had a problem with a long leash but now that I ride a hydrofoil I have found myself taking a powered dive off the foil and passing part way through the leash. That doesn't seem like a good thing.
I have many times used the long leash to tow boards while winding in lines to self rescue, or to tow a friend's raceboard (can't pick up big heavy boards with long fins hanging off and ride back to him), and to secure a rolled up kite when doing a long walk back to the car (having landed a downwinder and having to walk out).
Even when sailing in during a self rescue it's nice to have the board attached so both hands are free to work the bridle lines and get the best power out of the kite. My most recent self rescue involved a deflated main bladder so I had to roll the kite to pressurise what was left of the bladder and work the bridles and hold onto my board etc etc etc.
I think it's essential to have something to use as a tow rope and to secure stuff. Unfortunately modern harnesses don't have a pocket to stash a short length of line. Similarly I had the harness fail during the return of a coast run. I was able to finish the ride unhooked, but if things got really sketchy I could have jury rigged a harness from the leash line.
As for the handle pass leash, I have practiced reaching for it while being dragged by the kite. It's not necessarily easy, but it's not impossible. I find fear for my life tends to lend strength and flexibility I didn't know I had.
When towing people in the absolute worst thing you can do is get them to hang onto the leash. Leashes aren't that strong to start with, and having some boof head tangle his hand in the leash while you're powered up is a recipe for disaster. It's much better if they hang on to the handle with their hand and you ride slowly into shore.)
PS What do you call a downed kiteboarder sailing in doing a self-rescue? A windsurfer.