the way the spacers work is it replaces your hex and spaces out the wheel basically..but how you secure the wheel to the axle is by a barrel nut that screws onto the existing axle shaft(i think that is what you were curious about). the reason why someone would want the hex enlargers would be to fit some larger rims/wheels.
well that front shaft survived a 2S and 3S beating tonight. I think it will stay. I'm also impressed I didn't blow one of the axles out. I was jumping and bouncing under full throttle too. I am running the stock tires, they spin more then they bite so that may be why the stuff is living.