Problem is it's so easy to get a good sound out of headphones
Personally, I'm not sure I agree with that either. But maybe that's just because whenever I'm listening through headphones I'm always wishing I were listening through anything else.
I do think there is an issue with beginner users becoming accustomed to their sound via a given set of headphones if that's what they use first, after which it can be difficult to coax a similar sound from an amp without a good bit of tweaking.