[kbenson]

Sometimes a reply may be contingent on getting some other bit of information in the near future, such as a response from some person or entity that is required before the response will be useful.

In the end, not everyone's usage pattern is the same, and people end up using email for things other than direct communication (such as tracking future work items). While not ideal, for some it's easier than dealing with a completely separate application that integrates to various degrees.