From #1383:

It's very critical to add validation of directives in SDL since as noted here no one validates them correctly at the moment. That will lead to a situation very similar to the recent Allow interfaces to have no implementors. Since spec defines set of validation rules but no one implements them. Probably we shouldn't enforce Directives Are Unique Per Location until graphql/graphql-spec#429 resolved but all other validation should be implemented e.g. require the schema to contain applied directives, correct directive arguments, etc.