Yeah, I can't speak for the earlier designs, but I know that that's what the MM has basically looked like from the get go. And from what I hear, it's more like Steeda has been doing the copying (actually just buying and then reselling them; my "Steeda" pullies are ASP units).
The axle is positioned by upper and lower control arms. They're stamped steel and are prone to flexing. Aftermarket LCA's help tighten up the rear by better controlling the rear end.
The Pro3i's are available with poly at both ends (street) or poly on the chassis end and a solid end on the axle side (street/race). The solid end is better for performance, but will transmit a bit more NVH to the chassis. It'd be like when you swapped the shifter for the tri-ax. You get a bit more noise in exchange for performance. FWIW though, the poly will transmit a bit more NVH over the stock rubber too.