Oh dear, Crumbs, you seem either a little hard or thinking or are a GF. I'll be charitable and assume the former, and do your thinking for you.
Why disadvantage your child with a double barrel name?
What disadvantage? Statistics or studies please - the socially-paranoid rantinngs of those obsessed with net curtains don't qualify as evidence. 
Interesting - how can both mother and father have a good family name that is different to each other?
I have mine, he has his. Neither of us saw any reason to change our names on marriage, because each of us was content with the ones we had grown up with. Him with his father's, me with my father's.
Are they not a family?
Yes. Curiously, our sense of 'family' is not so fragile that it crumbles because we don't all have the same last name.
Has one not left their childhood family and fully entered their own family?
Was DH supposed to change his name to indicate he'd left childhood behind? Sorry, wait a sec, I'll nip across the hall and tell him.
Just seems silly and a disservice to saddle the poor unsuspecting child with an uber unfortunate name.
They seem to like it so far. And I'm comforted by the fact that at least they don't have ignorant twats for parents, and aren't saddled with a last name that would forever associate them with parents like that, so I reckon they're still ahead of the game.