... I wrestled with this too 9 or so months ago so finally went and got a proper surfboard leash. 5 seconds into session I hated it, kept getting tangled catching board, fin and legs, made me feel like a kook again. I persisted for 2 sessions and still hated it so threw it in the back of the car in frustration. I only did it becasue I had planned to go to Bali and try to hit some bigger waves where I believed it was necessary.
I ended up hitting some good ones in Bali and when I did get occasionally seperated from my board it was easy to get back to it especially since surfboards can drift a bit especially compared to a TT with straps that can catch currents and even end up staying upwind of you.
I reckon that if, unless your going into dangerous areas where you can't afford to loose the board or you don't have a way to recover it easily enough, then leave the leash behind and avoid the hassle.
I once watched my mate get rolled, board went through the lines and then kite relaunced dragging pilot and board into all sorts of mayhem so I figured better to have the hassle of chasing a board downwind than getting bent all out of shape and drowning.
But thats my opinion so check it out for your self.
cheers,
Robbie