The argument for not managing systems via packages
Although I haven't changed my mind in general, I think that there are arguments for configuration management systems like Cfengine and Puppet over packaging systems. Apart from their actual existence in usable form, one of them is that CM systems are going to be inherently more agile than packaging systems.
The drawback of packaging systems is that they fundamentally work on packages. This means that doing things via them requires a multi-step process; you must assemble your files, build one or more packages from them, and finally propagate and install the packages. Because they work directly, CM systems generally require less overhead to propagate a file (or to add a file to be propagated); you put the file in the appropriate place, modify your master configuration, and things go immediately.
(Because it does not have to build static packages, a CM system can also pull various tricks to make your life simpler, such as supporting not just static files but various sorts of templated files that are expanded for each specific system.)
A CM system also gives you a natural place to express the meta-information about what files get to which machines, letting you do so directly and without adding any extra mechanisms. A packaging system can express all of this (for example, you can have meta-packages for various sorts of machines and create dependencies and conflicts), but you have to do so indirectly, which involves increasing amounts of somewhat shaky magic.