Because Ubuntu will use the latest kernel on reboot and thus it will allow to delete the previously used kernel.
In case the latest kernel has a bug which can be discovered manu weeks after its first use, the user should be able to use the latest kernel which had no bug within a few minutes.
Because the kernel is critical, the user shouldn't have to wait for a bug fix in order to use its ubuntu box (bugs can still be open for months on launchpad).
Because Ubuntu will use the latest kernel on reboot and thus it will allow to delete the previously used kernel.
In case the latest kernel has a bug which can be discovered manu weeks after its first use, the user should be able to use the latest kernel which had no bug within a few minutes.
Because the kernel is critical, the user shouldn't have to wait for a bug fix in order to use its ubuntu box (bugs can still be open for months on launchpad).