Why need Debian?

[Tshepang Lekhonkhobe]

Hello,
A few times I have statements in the ubuntu website that there can't
be Ubuntu if there's no Debian. This statement seems to me like it's
only relevant to Warty, but why should they continue taking debian
sources, especially considering that Ubuntu's main is normally ahead
of Debian's sid? Is it the avoidance of effort duplication or is a
matter of maintaining compatibility or...?

Thanks...