It was very valuable, say, twenty years ago, when there were most programs were written or compiled using multiple closed source implementations of languages coming from competing companies. There were real economic incentives for the implementations to diverge from each other in ways that harmed the larger ecosystem. A formal spec was a forcing function to make those implementations compatible.
Today, programmers simply won't use a language whose implementation isn't open source with a very permissive license. This makes it very hard for an organization to deliberately make the implementation incompatible with others because other organizations and users are able to either avoid the incompatibility by forking the implementation if they don't like it, or making it compatible by using the implementation if they do.
I still think it's very valuable to have a committee with members of all implementations that helps drive consensus for where the language should go. But the document itself I see as secondary to that human process.