Just did some quick math. Assuming the cases are constructed the same way, roughly...
There's about 60 cases per lb. for brass .45 Colt. That works out to 0.2667 oz. per case.
The weight ratio of Aluminum to Brass is 168.48 lb./cu.ft vs. 535.68 lb./cu.ft. or 0.3145:1.
So an 0.227 oz. brass case will weight about 0.0839 oz. in Aluminum.
So in a 6-shot revolver or speedloader you'll save about 0.5484 oz. That's 239.9 grains of weight savings per reload. So about the same as having (or losing) one FMJ .45 bullet.
If you can put on your pants and tell by the weight whether you've got one .45 FMJ bullet in your pocket or not, this might make a lot of sense!
...
To "flesh out" jmorris' point: The human bladder can hold up to maybe 17 oz. Let's say if you've REALLY gotta go there's probably 15 oz. in there. That's about a pound of water weight.
If you go "tinkle" you'll drop TWENTY NINE times more weight than you will changing to aluminum cases in your .45 Colt.
As the scientists would say, "
Naww, gonna be like a fart in a hurricane. Don't waste yer time, son!"