Was Dumbledore aware of house-elf magic? If he was aware of house-elf Apparition, he should have gone to the Cave with Dobby. It seems nobody in the story made use of elf Apparition fully (it isn't abuse, as making use of Hermione's magic wasn't abuse).
For instance, Harry might have gone to Hogwarts/Gringotts in the final book with Dobby/Kreacher (along with Hermione,Ron) avoiding much trouble.
What are the limits of house-elf apparition?