If I am reading correctly, I would save myself cash and a headache by adding an auxillary power outlet and just plugging the ZUMO directly to the audio jack?
It seems the only thing the interface does is charge the device as I would be using XM on trips regardless.
Is there any valid reason to justify the interface cost?

Thanks for your advice in advance.

The fairing mount or handlebar mount is all that is needed to charge the device. The cradle has the power cords +/-. The NIM harness allows the spoken directions to interrupt and come out the bike's speakers regardless of the audio source. I can be listening to a CD, radio or iPod and when a turn comes up, the garmin will interrupt, speak, resume playing CD, radio, or iPod. The NIM still requires the use of the aux for MP3/XM from the garmin.
Using only the aux input, will require the radio to be set to aux to hear spoken directions.

I understand that part, but I will only use it for nav and XM.
A aux input cable is by far the best route for me it seems.

if I remember right (and it has been awhile) the gps does not recognize that it is on the bike if you don't use the interface and that the antenna for the xm plugs into the interface..sooo, if you just plug it in you cant use the xm features...

I do know you can plug it in to a vehicle and use the gps and/or the XM. So I would think it should work otherwise.

I will just hook it up and see I reckon.

To be honest, if I did not get it new for 450 bucks, I would not have it.

The NIM also allows for the use of the Harley headset if you plan to use that feature.

Yes that is my setup and by far "good enough". I have used it this way for 5 years.

Recently I added a new Sena bike communication helmet to helmet system. Zumo interfacing with it can add the audio to the headset using bluetooth. On a trip I switched back and forth between the cable and bluetooth with ease.

Buy the cable, save the money. 3 bucks and you are rolling. I ran mine down the handlebar on the left side, under the fairing and back up just under the radio aux port.

Thank you for your input! Silly you have to use the aux either way.