I rebuild wheels with used spokes all the time. However, these are on wheels that I originally built, so:
1. I know that the spokes are good quality
2. I know that the wheel was built to a high standard, correctly tensioned and well cared for
3. I sort the spokes into "inner" and "outer" sets when disassembling the wheel, so they go back in the same orientation

All of these might not be true on a random set of used spokes.

When I worked in a shop, we would cut out old spokes with cutters and dispose of them. It wasn't worth the time to carefully disassemble the wheel to save the old ones.