If one learns to play by ear, one will be FAR less likely to ever learn to sight read.

If one learns to sight read, it is less likely that one will learn to play by ear.

The question becomes which of the two above is easier to go against?

I think it's probably easier for a sight reader to learn to play by ear than the other way around. Given that, it's not a bad idea for a teacher to suggest that a child not first learn to play by ear.

Why? ...because the very best option would be to be able to do both equally well.

I know a few people (including the best fiddler I've played with) who, as youngsters, faked their way through orchestra because they could mimic well (by ear). But when push came to shove, the door finally slammed on them when they REALLY needed to be able to sight read.