The use of students' L1 in a second language class

Over at Ben Slavic's blog there has been a discussion of using English (L1) in a second language class.  We all know L1 is to be avoided. It says so right in district, state, national standards, research, methodology textbooks, etc.  But, it's that 800 pound gorilla. How do we get rid of it? The students speak it. We speak it. It seems so easy to just slip into English for a few seconds, share a story, explain something, go back to Spanish...

Ben is discovering that, as he experiments with a radical expulsion of English in his French classes, his students are paying more attention, they are using less English themselves, and in fact they are happier and more engaged. We always slip into English thinking we are adressing *their* needs.  But, what if we aren't?  What if English just kills it all?

What if the answer to kids speaking in English is simply for the teacher to speak more in L2? (while maintaining complete transparency, in other words staying always in bounds with the class.)