Single implementation is important for consensus systems. Look at e.g. the spurious dragon hardfork.
The problem with multiple implementations is that if there is a tiny difference in behavior between them, you can only find out after damage has been done to the network via a split/fork.
I think the idea is that the protocol is resilient to differences in implementation. A bad actor, could for all intents and purposes, manipulate the implementation to their advantage. An implementation, reference or not, is merely a convenience for users who do not want to write their own clients (which is essential for adoption).
In fact, last year an attacker launched a denial of service on the main Ethereum implementation, and the network barely hiccuped because people just switched over to one of the others.
The problem with multiple implementations is that if there is a tiny difference in behavior between them, you can only find out after damage has been done to the network via a split/fork.