Some packages aim at fast moving targets like spam filtering and virus scanning, and even via using updated virus patterns. This doesn't really work for the full time of a stable release. The main issue of volatile is to allow system administrators/users to update their systems in a consistent way without getting the drawbacks of using unstable, or even without getting the drawback for the selected packages.
Besides being built for stable (lenny), all the dependencies are satisfied inside stable, so not external or additional backported packages are needed (IIRC)
404 Not Found, not available in Squeeze
While running aptitude update, you get
... http://volatile.debian.org squeeze/volatile/main i386 Packages [ERROR] 404 Not Found [IP: 220.127.116.11 80] ...
DebianVolatile has been superseded by squeeze-updates since Debian Squeeze. read the instruction on StableUpdates.
http://www.debian.org/volatile/ - Debian-Volatile official page.