Outer Limits 4x4 Board

I've bought a plug to hard wire a permanent 12 volt connection in for my GPS which I was going to piggy back the power for from one of my power sockets in the front cabin of the car. The wiring diagram shows to connect it direct to the battery and fuse the +ve. Is this really neccessary for an accessory that will draw stuf all current?

Absolutely - I always wire direct from the battery with 6mm wire, with inline fuse.
Then you can feel comfortable using it for all sorts of stuff, not just the GPS

Always fuse any +ve supply line - the issue is not how much the device can draw, it's how much a short circuit can draw.