One of the shifts of the common core is "Reading, writing and speaking grounded in evidence from text, both literary and informational." Is it always warranted that a student cite using MLA format? No. Teaching students how to informally cite text to support what is explicitly stated and to support inferences is very important as they analyze texts for multiple purposes.
This document is a great visual to teach and remind students how to cite the text to support answers.
Standard W.4 is covered here if the students are writing their responses.