Learning a second language should be MANDATORY for every ESL teacher.

Understanding the difficulty of and techniques necessary for second language learning is critically important if the ESL teacher is to successfully assist his students in their learning process.

An EFL teacher in China whose only language is his native English can neither appreciate nor understand his students' struggle to acquire English. Much of the disdain that FTs seem eager to cast upon their students' language learning abilities would be ameliorated if the teacher had experienced his own difficulties acquiring a second language.

Just as Kobe Bryant or Tiger Woods would likely be poor teachers of their craft given their natural abilities, native language ability is inadequate preparation for teaching second language acquisition. If you have not learned a second language as an adult, you are unable to understand this limitation and are also unable to critique this proposition.