USB debugging is for connecting a device to another device, ie an Android device to a PC. It does not, however, give an Android device the ability to run ADB locally to connect to another Android device. My situation is, I have a TF300T which is a tablet with a dock, the dock has an actual USB port and functions like a USB OTG adapter. I used to be able to connect other Android devices to the USB port on my tablet and, using a terminal emulator, run ADB commands to the external Android devices. Since upgrading my tablet to 6.0 however, I discovered that I could no longer perform that task. Some quick Googling turned up a couple of places asking about this exact function, and someone on stackexchange posted a question about that exact issue, however they never got any response.
Why Google felt it was necessary to exclude the ADB binary from 6.0 is beyond me. Sure, it's something that is probably rarely used by users, but for those of us that -do- use it, it's not cool that it's suddenly gone.
I wonder if it wouldn't be trivial to extract the binary from a 5.0 device and see if it would just work under 6.0...