It's more for the crucial spin factor then weight reduction. More RPM and less parasitic loss.
My metal matrix shaft is lightweight indeed, but the aluminum shafts have to be larger in size thus having more weight compared to mine, but lighter than steel.
Wish there were titanium slip yokes available in the market for Chrysler. None that I have seen and probably not worth too much in weight other than strength which can be achieved via a billet unit.
By the way, as far as costs, my friends 3-1/2" Aluminum shaft cost $544 with a Sonnax billet slip yoke, Spicer 1350 U-joints including shipping and all and that's not too much more than a good Chrome-moly shaft.