What are the advantages of the back to back belt & pinion systems? Except for slightly better accuracy I don't see the distinct advantage over belt and pinion. I'm sure there is one, its just not obvious to me. Also, both of the examples use two two sets of idlers (one big, one small) anyone know why that is? Both look like this from the side: [_oO*Oo_] Again seems like one set of idlers would get the job done. Thoughts?