Spool Clamps can be a very valuable tool to have in your tool chest. They are specialized tools, but are incredible when they are needed for certain tasks.
If you purchase enough of them to say, to clamp an acoustic guitar back to the sides, you will need at least 24 to 32 clamps. That can be a lot of bucks, especially if you can make them yourself by investing a small amount in hardware and using some scrap wood around your shop.
I'm sure you will find many tasks where these handy little clamps can come to the rescue where other tools prove to be too big or not to adaptable to the task at hand. While they are often used by instrument repair shops, they can be very useful when building new instruments as well, and that is almost exclusively how I use them.