Usually certification does not required for a gifted artist, because his abilities are self-evident, but for low-skilled craft man to prove that he possibly could not fail. It is a trick to not to show your abilities each time, because it is extremely difficult to you - think a certificate of sexual ability for impatient - for a healthy man it is not a problem to "prove his ability" =))

mr. Nabokov, for example, didn't got TOEFL before he wrote his "Lolita". TOEFL is required only for pidgin-speakers, like me, and after some amount of practicing it become useless.