I agree with susan if using L1 helps the students with learning a foreign langage then use L1, on the other hand if it hinders the students'learning, well not using L1 appears to be a better choice. Personally I use French to explain grammar points because we compare the English language to our own language and review the French grammar at the same time. For very young learners I give the instructions in French because it saves time and when my students have a good grasp of English we just use English and "ban" French!!
Personally i think that some kids can lose motivation if we use a language they don't understand a word especially when they're beginners.
