The problem I see is with the use of the word "clearly".
For example, some email readers organize the emails in a specific way (directory tree, database, etc...). You can argue that the format of the data being specific to the application, there is no point in keeping the files if the application is removed. On the other side, removing these files erases a long email history that could not be reconstructed.
Of course, configuration parameters specific to the application are good candidate for automatic removal. But these are usually small: not removing is no issue. If these are not small, then they do contain some valuable data. I would want a package removal process to fall on the safe side and not remove them.
Then there is the case of the nazi sysadmin (I met some) who decides that a specific application is useless crap and removes it; some users rebel and demand a reinstall. If that has never happened to you, you probably have never worked in a small high-tech company (in large corporations, to rebel is usually pointless). In other words, the user may not be aware the sysadmin decided to remove the package and might object to its removal.
At the end of the day, the only clear rule that works is: what is owned by the user is his and shall not be removed by third parties, no matter how well intentioned. A sysadmin who removes files owned by me is going to hear about it...
BTW, debian dpkg does not remove configuration files by default (it has an option for that) and that only covers system configuration, not users.