I doubt there is any usefulness in cordless dremel tools. I bought a store brand cordless rotary tool from Menards a year ago and I knew immediately that it would be trouble. After the first charge, each battery would run for about 5 minutes on high speed. The highest speed setting was as fast as a corded version’s lowest speed. It did however come with a full accessory pack; lots and lots of cutting/sanding bits plus a flex extension tool.
Yesterday, I bought a Black and Decker corded rotary tool. The same $20 that bought me the cordless crap tool, bought me this wonderful tool. The black and decker tool didn’t come with any real accessories, just a handful of cutting/sanding bits and a wrench. I was surprised to find that nearly all my old accessories were compatible with the new tool! Even that fancy flex extender. So now my new tool is happy in a new box, surrounded by more sanding bits than it will ever need.