There are many ways to get Exotic items as well and that doesn't stop Xur from selling random Exotic weapons. Once again if a merchant can sell the top Tier Exotics in the game then there is no reason the same merchant cant sell limited Etheric Light. It should just require more time invested than the 23 strange coins required now for primary and it should be limited to 1 a week per character. I would say 50 strange coins and limit 1 a week per character fits the current strange coin inflation.