I have bought a lot of these dummy plugs over the years, usually adding a few to each order from Thomann. I mainly use them on my more expensive equipment and anywhere I have unused jack socks that are exposed to dust. In particular I plug the sockets on my Yamaha 01V mixing desk as these are extremely exposed in my studio. Not the most 'fun' purchase but one I consider indispensable to protect kit. The only downside is that they are not offered as a bulk purchase with a discount :-)
Possibly even more important than the Neutrik NDF and Neutrik NDM covers - this keeps dust and smoke out of the sensitive switched 1/4" connectors on my PrismSound Orpheus. Absolutely indispensable - well, if you don't want crackles!