I have an kayak spray skirt that I have had for years (used both on Ks and Cs) that has this functionality and it works well. Although KNeal hated it and swore it leaked like a sieve when he used it to paddle Slippery Rock Creek with me!!
He even made me change skirts with him and everything!!! Personally, it worked for me that day and I think he just was trying to get out of my Zwo he borrowed to stretch.
Anyway, I never used it to "knee off" but my son is currently using it on his K and it provides him with another system to "grap and go" with - if he needs to exit (he's only 9 and too small to "knee off").
I like the fact that it is closer to the seat so he doesn't have to reach as far (kids have shorter reaches) and is much easier for him to find when upside down (he knows where his legs and the skrit are). Also since we toik the time to set it up right even the lightest tug pulls the rand up enough for him to get "flushed out" of the boat. Which is good since kids are not all that strong, don't have leverage, and have a higher "freak out rate" while upside down. He had a heck of a time with a Mt. Surf Bomber and had to swim out of the tunnel - proving its good to be skinny and not fat like the old man.
Oh, we were testing skirts for him in a pool so he didn't freak.
The one comment I heard from others as to the system is that the strap could "foul" on a strainer, etc. and therefore could place the paddler in more danger.
However, I think as with any system it has both pros and cons and you have to take into account the user and the situation of use.
Personally, I never had a problem with that skirt and it will continue to be the skirt my son uses on his k as he learns the way of class I-II.
I hope this helps!