I have 2 different rims that need re-spoking as the al nipples have corroded and cannot be turned so it is a lot of work to rectify - and often the spoke will break or twist before the al nipple breaks free so you can't just re-use the old spokes. In my opinion, they are one of the worst cost/gram savings you can make as rims are too expensive to justify the grams saved - that said, if you're after a rim to last a couple of race seasons, the trade-off may be worth it.
Note an al nipple weighs about 1/3 gm and a brass about 1 gm so for a 32 spoke wheel you'd save about 20gm/wheel.