Poly Bushings issues

Okay, okay, we all know that you're supposed to use rubber bushing in the lower control arm because the poly bushing will make noise. However, what actually causes that problem?
Here is a part for the rubber bushing
http://www.americanmuscle.biz/34572_lower_control_arm_pivot_RUBBER_p/34572.htm

And this is the part for the poly bushing
http://www.americanmuscle.biz/34571_lower_control_arm_pivot_poly_p/34571.htm

Notice that they are different. Could the reason poly bushing in the lower control arm is not used because people are use the wrong pivot?
Anybody using this poly designed pivot with a poly bushing and it still make noise and which type of poly bushing are you using?