And right there, in the same quote, it tells you that you don't have to - that many typical half-elves will talk, dress and act exactly like their adoptive culture. Why are you against people playing one of those half-elves?
Because, what's the point? Why be a half-elf if no one actually knows it? Why not just play a human if that's what you're going to play anyway?
If your character acts human (or I suppose Elven, although that's generally not the problem), dresses human, and is in every way human, why doesn't it say "human" on the character sheet.
Oh right, number bonuses. Go go powergaming. Can't be a human if I want to multiclass, so, guess I'll just act human, no one will know the difference and I'll just write half elf on the old character sheet.
It's the worst kind of role playing. You're not actually PLAYING the role you have. The player is giving no indication, no information to anyone at the table that allows the other players to have an accurate image of his character. Everyone else thinks that PC is human. They are caught completely by surprise by the revelation that he isn't human, when the characters should know this information. It shouldn't EVER be a surprise.
((Unless, of course, it's deliberate - that's a whole 'nother ball of wax, but, we're not talking about that))
The character is being played so poorly, portrayed in such a manner that lacks any indication of what the character actually is, not because of some deeper plot involvement, but because the player is too lazy to bother putting the thirty seconds of effort it takes to differentiate himself from the guy standing next to him.
And you guys are applauding this. That's what totally blows my mind. That this isn't seen as a bad thing.