Originally Posted by Drew Eckhardt

I've been using the alloy flavor.

To speculate I'd guess the threading allows you to aim for the top with the same error tolerance for long (run out of threads) and short (don't make it through the rim, load the nipple in tension, and break it) spokes provided that you're using double-walled rims where the excess length won't hurt.

With brass nipples you can get away with being a lot shorter