Apples to oranges comparison..or is it an blueberry to eggplant comparison.
Ubuntu's time based development model strongly leverages Debian's much looser development model. It could be argued that the very loose development model that Debian has is a pre-condition for Ubuntu to exist at all. You really can't hold up Ubuntu's development model by itself as a workable model in-and-of-itself. To understand the how's and why's of the Ubuntu workflow, you have to incorporate Debian's development model as Ubuntu is a derived distribution which rebases from Debian.