In small surf, why would you need one?
In big surf, you don't want it if it is the standard mount at the middle type.
Somewhere in between come the choice, where you may come off, have to roll off to avoid another water user or it is big going out and you time it bad and roll the ski rather than get smashed.
Usually if you roll the ski voluntarily, it is easy to grab the footstrap and hang on.
You can then keep yourself forward of the middle with an outstretched arm and the ski will stay pointing into the wave.
But coming in, if it is big, and you come off, the ski will always end up towards the shore anyways and you really don't want to be attached to it in those circumstances.
I often head out by myself, so I will use a leash and mostly forget about it and paddle through the surf in with it still on.
If it is big enough that I stop outside for a bit of a look see and go for a gap to come in on the back of a wave between sets, then it is leash off always.
But in generally friendly conditions, who cares.
I do so want a rear mount leash though.
I was heading back out after stopping for a drink and there was a vicious little chest high shore break.
I went to jump in but the leash got jammed in my pedals and I jumped out again.
Then a whole set come through and I waited and waited some more and then jumped in and paddled past the break.
My leash again got caught up in the footwell and my right foot couldn't get under the strap or work the rudder.
So my self centering steering done the work for me and after the last breaking one I started sorting out the leash and tried to get my foot in the properly.
I fell out doing this as a non breaking wave rolled under me, much to my paddling partners amusement.
He was waiting out there since the begining as he ran no leash and simply jumped in the boat during the initial null and paddled out no probs.
If I had a rear mounted leash, this problem simply would not exist.
I could have hooked it up standing holding my ski, or clipped it close behind me, jumped in and gone.
Follow the path of the independent thinker. Expose your ideas to the dangers of controversy. Speak your mind and fear less the label of 'crackpot' than the stigma of conformity. And on issues that are important to you, stand up and be counted at any cost.--- Thomas J. Watson