Center of car net?
 

Are the nets that run down the center of the car effective as a side head restraint? or are they more to keep your arms from flopping around?

Arms and shit flying around inside the car, and things which enter the car.

If your head was to "flop" to the side enought to make use of one of those nets, it would be the last time you did. Hons device or a under helmet neck roll are for keeping your head on straight. Window and center net is for hands and arms. You don't need either one.

Shit looks dope though. Ricecar dope.

I actually bought a Necksgen a couple months ago.

I think you seriously underestimate how much the head moves in an accident. A side net (without a halo seat) when properly installed is primarily designed to restrict head movement. It also helps to restrict seat movement in a big accident, which is why you'll also see them with halo seats.

What's interesting to me is that even closed-cockpit cars (ala Rolex Prototypes) have gone to twin "center" style nets in the car.