I normally fish with low end equipment so I don't worry TOO much.
I secure the extra rods in one way or another but don't worry about the one I'm fishing with as I'll get out and wade with the kayak staked out or tied to something or tethered to my beltloop. For that reason I couldn't (wouldn't) fasten a leash to my rod.
Ninety percent of the time or more I'd be able to retrieve my rod/reel as I am in water that probably averages 3 feet deep. Crossing a channel or hole would be about the only times it would be too deep to easily retrieve.
I do see where this rod leash would come in handy for fishing deeper lakes/bays.
"Only the half-mad are wholly alive." ~~ Edward Abbey