Well, debian does backport all relevant security bugfixes. That aside, there will always be newer, bugfixed software. Debian stable focuses in being a bugfree distribution, not a distribution comprised only of bugfree software. Which means there are, ideally, no version incompatibilities nor out-of-the-box misconfigurations. Given Debian's almost inconceivably big repositories, that's quite the herculean task.
For the record, KDE has been stuck in 4.8.4 for about six months, since the freeze started, but since 4.8.4-2, all bugs that initially affected my machine seem to have been ironed out. Whether that's because they have implemented upstream bugfixes or because they were actually Debian bugs to begin with I can't really say, but if you campare it to, say, Fedora 17's or even Kubuntu 12.04's KDE 4.8, you'll realize how marvelously quirkless Debian's KDE is and why it pays to have stabler distributions.