There are two reasons why Ubuntu includes so much software. First, they basically get it all for "free" from Debian. It's all packaged, and Debian does at least basic QA on all packages. Second, Ubuntu wants to allow users to run a lot of software. It is important to realize that support contracts for Ubuntu only cover a small percentage of the available software.
The reason why I keep bringing up business use is because that is the group that pays Canonical for support. Ubuntu has a great user community (these forums are a tribute to that), but the development community could not support Ubuntu alone. That means if Canonical cannot support Ubuntu, then it goes away. There has to be a revenue stream for Canonical, and that means selling to businesses, which would not tolerate a rolling release.
From a financial point, a rolling release would bankrupt Canonical, and turn Ubuntu into a package repository. The only improvements would come from upstream, which is exactly where Arch is.
David Ahern from Cisco had a wonderful presentation at LinuxCon last month about the difficulties of standardized software. Most customers look for five year platforms. That means no software changes (security patches might be acceptable) for that entire time. Most Linux deployments are commercial, and over their lifetime will not be current.
Debian is a tremendous successfully community-developed distro. They don't sell software or support, so they differ substantially from Red Hat and Novell. Canonical lies in the middle, but I don't think that Ubuntu could survive (in the same manner) as a community distro; and that is precisely what a rolling release would do to it.