Multiple implementations is a great way to highlight issues in the spec vs reality (assumptions, unspecified behavior, etc). If you only have one implementation, the implementation becomes the spec and you end up with people targeting compatibility for that rather than a specification.

This happened with Cisco routers back in the day, Juniper had to add a flag to make a routing protocol compatible with Cisco because Cisco implemented it wrong but had so much dominance that Juniper had no choice if they wanted to get in the game.