Nationals of countries with spanish as the official language when taking a flight test in Spain for a spanish licence with a spanish examiner are generally issued the SLP-6 based on that flight test provided the RT has been satisfactorily conducted in the spanish language as judged by the examiner. In that case the SLP will be entered into the spanish licence. Not so in my case, I had to take the test in order to provide official proof of spanish language proficiency (SLP) and I keep the FTE-issued certificate as a separate document.
It is definitively worthwhile to obtain your SLP and it will open up opportunities that you would otherwise find inaccessible.