We only buy EID pair tags now as only using one set makes numbering and accountability easier. Any lambs leaving our place are tagged with an EID and non-EID just as if we were keeping them.
As I understand it, if I had only put slaughter tags in and they went to market, if someone wanted to keep breed from them they would have to replace the tags.
As there is very little cost difference I'd rather tag them with two to cover all bases.