I think that learning how to tune their instrument by ear is important for anyone serious about developing their own musicianship.

I don't think anyone is truly 'tone-deaf' - after all, we can all hear tones! The only difference between people with 'lousy ears' and people with 'good' ones is that the people with 'good' ones have spent lots of time and energy training and using them, in my opinion. Either in this life or in a past one, not to offend anyone's religious sensibilities. (Actually, it was trying to understand why some folk are born with 'good ears', or 'perfect pitch' that convinced me to accept reincarnation as at least a possibility!)

But back to learning guitar - I'd 'test the waters' by introducing tuning by ear at the first lesson to ALL students. But I certainly wouldn't put a student off by insisting they learn to tune it by ear if they didn't WANT to learn how. It's not an easy thing to learn for a novice, and if people are not highly motivated to work on it it can really be frustrating. I believe that loving and enjoying music - and music lessons - is the first priority. So I'd let them use the tuner, and sneak in ear-training in other 'fun' ways during the lesson. Like you said, at least on guitar people can get away with not having a well-trained ear.