Full auto and cool factor have nothing to do with it.
The "FA" BCG is what the system was originally designed to work with. The other types (SA, non-FA, half circle, whatever you want to call it ) were made as a result of manufacturers bowing to political/legal/etc. pressure.
The BATFE have already ruled that FA BCGs are legal to possess, use, etc. in the US, but despite this, some manufacturers still persist with this idiocy. Although some individual states (Commiefornia comes to mind) do have separate legislation I believe.
FA BCGs are no more special/rare/expensive than any others. I believe that we should not be referring to them as FA BCGs, as this plays into the anti's hands when they are looking to ban/restrict stuff (can you imagine Wendy, "it has fully automatic parts in it!"). We should call them Full Weight or Full Circle BCGs, or something similar.
Regards.
Mark