I believe that it is 1lb rotating weight saving equals a 1lb reduction in rotational inertia + the 1 lb static saving, so would require a 2 lb reduction in static weight to provide the same reduction in overall reduction in inertia.
I think it only matters when you are accelerating either horizontally or going up a hill (force against gravitational acceleration).
Hmm I once would have been able to explain this properly, but alas no longer, I just realised I am not really a proper engineer any more.

davida