I'm a bit on the fence about the need for something like this. If I set my output levels correctly and position myself near my speakers, I can pretty much get a chord to sustain for as long as I could practically think of a use for it. I couldn't find a use for the hold function on my dear departed GT-8 either for the same reason.
Eagerly waiting for a looper though. It would be kind of cool if the loop would also work on a momentary basis, i.e. step on a pedal to start the loop, then when you take your foot off, it deactivates.