Allowing developers to create their own semantics as they see fit does not create a more semantic web.

The whole point of having semantically meaningful documents is so that machines can better understand them. Having a system where each developer creates their own "meaning hooks" is worse than useless - its a waste of time and bandwidth - and leaves us in the same position we are in today where a machine ignores markup and tries to infer meaning from context.

Standards and convention are important.

The only way to create a truly searchable and meaningful web is for us all to use an agreed upon set of tags that add meaning.