package:dpkg in situations where a user cannot interact directly with dpkg (debian-installer is one example of this though conffile prompts shouldn't normally happen there) conffile prompts currently result in a hung installation process, this is not a good thing.
since debconf seems to be the preffered method of communicating with the user and already has mechanisms in place to allow for situations like D-I and for completely non-interactive installs it would seem more sensible to use it.
