I have two guitars fitted with GK-3 pickups that use 13-pin cables - on one of them, the pickup eletronics is built in. And as these guitars are customized Ibanez high-ends, there's no way I'd sell them :)
This little converter is a must have if you want to use new Boss GM digital cable synths - and it does its job. The added benefit is the guitar-out port so you can feed the 'normal' signal to another effects unit or a sound card for future processing.
It needs its own power adapter - something to bear in mind when building a pedalboard layout.