i am on my second year wave riding and will loose my board while out at some point. while a twin tip will just float in the water, a surfboard will skate across the water and wind up 30 meters downwind before i can even recover from the crash. i don't plan on jumping my surfboard. that was the real danger of leashing your twin tip that i can tell, if you lost the board in the air, when you landed you had a missile bearing down on your forehead.
used to leash my twintip as a beginner, got rid of it when I started doing big jumps so when u kick it off your feet if won't come behind.
didn't want any leash again...
recently I bought an SRT 5'11 and lent it to my mate to go to a wavespot, lots of rocks (like at any good wave spot) at the beach offcourse, and it also ended up with the tip of the board against a rock.
He got it fixed and got me a leash, had a short ride on it yesterday in 35knots. I like it!
No more forever bodydragging to get to the board, no more having to stay away from the shorebreak (we got a forbidden military kitespot here where there's no beach at high tide, just one big rock wall. Definitely didn't want to get too close to there)
-when u loose your board the leash can get caught either behind your harness or around a leg, pretty annoying when u want to get up on the board and somethings holding u back (especially with a huge wave coming hehe)
-again the leash can get fiddly when jybing, but i'm still learning to jybe so drop it a lot. I think this won't happen often.
-u need to know where the board is after the crash! if its towards the wave, yank the cord so its either downwind of you. If you don't have time for that, either hold your hands/arms in the direciton the board is, or if u don't know at all just protect your head with your arms!
usually with the leash the board won't be too far and u can either give it a yank towards u, or it will already have drifted more downwind and past the wave of yourself... no danger!
Positives: no more loosing board at all, if it gets nasty near shore. Bodydrag out to sea with the board hanging onto your leg!
no more worrying about the board getting lost in big surf(I know how fast they go with the waves, not fun!!!), u can put all your focus at keeping the kite in the air while u get tumbled in the surf
Also even when it's not dangerous, I find in wave conditions its almost always impossible to bodydrag upwind because the waves and wind (and kite) pull u waaay downwind and the board usually only moves in the direction of the current and waves (wich is sometimes upwind).
PS. my mate got me an o'neill leash, I think the length is about 2 meters? I don't jump higher than about 3 meters either with it (not really jumping, just coming fast towards a wave and getting popped into the air. but always land it soft offcourse! no trix!!!) I think the chances of getting the board yanked towards yourself are minimal with the kind of jumping that happens on this kind of board.
Conclusion: big surf? a shoreline that u don't want to come close to? get a leash but be well aware that the board will be in your vicinity when u crash!
there's little risk of the board slingshotting against u since u don't loose it during jumps, but a wave can get it going towards u pretty fast