I have a few acoustic instruments, all of which have these same pickholders. I can't live without them. Many of my fellow musicians lose far too many plectrums after putting them down and forgetting where they are. Not me.
That does mean I am beginning to find out the shelf/usage life of a plectrum. Some of them I've had for years and they're starting to wear out and get grooves cut into them.
The only slight disadvantage of these holders is that if they're not completely wedged full, they can rattle when you hit low notes on the guitar, which is no problem live but when you're close miking in a studio it can easily be heard. It is just a bit of plastic and a spring after all, so you can't expect it to be highly engineered and silent, and it's a little gadget I wouldn't be without.
That's all really. I even have 2 on one of my guitars as I sometimes use different size plectrums.
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