For security and reliability, being open source means there are more eyes on the codebase. Bugs in the code can be more easily found and more quickly patched.

This affects network evolution too, because it means anybody can make changes to the code. If you don't like some particular node software, you can always write your own or use somebody else's implementation. This makes it difficult to change the protocol because one group of developers can't push updates out onto all of the different node implementations.