Never been convinced about that reason as I found the 370 rear wheels fit fine on the front (I've tested in the event of an emergency)
What it was useful for though was taking the disc off the hub - just remove the sleeve and then screw in the bolt that will separate the disc from the hub
Otherwise, removing it completely is no issue.
Just what I read Colin and I cant think of any other reason, you did try the wheel without your spacer and with the stud in ?
off 370z.com "I began with removing the front wheel, and then removing the "idiot bolt" on the rotor that prevents you from mounting the rear wheel to the front of the car. I forgot to take pictures of the removal of the caliper, but the DIY on this forum is great."
"It just unscrews. It's there so u don't accidentally put a rear wheel on the front."
"That bolt is not needed. It is there to prevent some unknowing person from putting the back wheels on the front."
agree it can help to remove the disc but why would the rear not have one if that was it's purpose ?
Not sure if the US 350/370 specs may be something to do with it noting the US forum you link to, but in answer to your query GUNs was driven around on a parking area on full locks trying a rear wheel bolted direct to the hub to make sure there would be no rubbing etc in the event it were used. See reasoning for using a rear wheel on the front here: http://www.350z-uk.c...ge__hl__spacers
It was due to that thread I bought a 350z space saver