The only way to become fluent in a language is by being immersed in the language.
Forcing students to speak only English in class requires them to negotiate the learning process in English.
Students speaking another language distract other English learners.
English only is part of effective classroom management.
These are all valid arguments for an English only policy in the ESL / EFL classroom. However, there are certainly arguments to be made for allowing students to communicate in other languages besides English - especially if they are beginners.
Providing or allowing for explanations of grammar concepts in learners' L1 (first language) speeds up the learning process.
Communicating in another language during class allows students to fill in the gaps, especially if the class is large.
Allowing some communication in learners' L1 establishes a more relaxed atmosphere that is conducive to learning.
Translating vocabulary items is much easier and less time consuming when other languages are allowed.
Committing to an English only policy in class effectively turns the English teacher into a traffic cop.
Students are limited to learning important concepts by their English language skills.