I'm with you on this. It's always seemed weird to me. I mean, I do comment on people's attractiveness to other people, but it seems odd to seek out their partner -- the one person who you'd think least likely to need alerting to the person's attractiveness -- and tell them. Complimenting them on their choice seems a bit weird, almost patronising -- I mean, if their partner's right for them, they don't need somebody else to say "Oh yes, good choice, I approve". Or they shouldn't need it.

(If someone tells me sion_a is attractive I'll most likely beg them to tell him that, since he seems to have trouble believing it...)