nickrose:

I'm guessing it was for the convenience of the wireman

Wrong !!

I’ve worked out what the extra diode is for. It’s designed so that if you plug into the TAP jack, you won’t get a switch closure on this jack when you hit the T+R.

If you rely on this feature you’ll have to wait for a software fix. Note that the Digitech FS300 does not have the extra jack, so this issue does not arise.