i personally think that the english only approach is the worst approach. it makes students whose L1 isnt english feel inferior to the native english speakers. it makes the students feel as if their culture and language isnt important. also they wont be able to get the learning they need to know english fluently or keep their L1.
Yes, ignoring the one or two non-native English speakers in a regular class would be insensitive and a lot of time can be spared by explaining in the student's native tongue, if possible.
That said, I think the context of this English-only policy is when ALL the learners are non-native speakers in an ESL/EFL class. When an English-only rule is not enforced, especially when all the students have the same L1, then it's far too easy for students to fall into their more comfortable language and the opportunity to have them use the English they know is easily lost.
My German teacher in 1st year uni taught us with German-only in my absolute beginner class. Many of us didn't even know "Guten Morgen" on day one. I personally knew nothing. We picked up everything by context, repetition, and because she taught us what we needed
to say. Creating the German-only atmosphere was really effective for me since I'm shy and resorting to L1 was always a temptation.