It is common to find single applications that integrate several services that could have sensibly been provided with separate applications instead. How much integration is smart is, of course, a matter of opinion. I think integration makes sense if the things being integrated are closely related and can reasonably share application features. For example, Thunderbird supports both email and NNTP newsgroup access. This makes sense because both mail and newsgroups serve a similar purpose. Also both systems can benefit from a unified address book and common message folders. In addition, both systems support similar kinds of message content.
What about instant messaging? Many people today use IM as a major form of interpersonal communication. In some communities it is replacing email as the communication method of choice. IM systems such as Jabber even support asychronous messages; the recipient of a message need not be logged in at the time the message is sent. One can easily see the sense in using an IM sytem for informal conversation and traditional email only for longer, more formal, and more complex documents.
Given this observation, does it make sense to build an integerated email+IM client? Again both systems could benefit from a unified handling of contacts, and it might be nice to store chat session logs in folders alongside email messages. Indeed, Google appears to be headed in this very direction with Gmail and Google Talk. However, I do not know of any desktop email+IM application. Is there one? Should there be one?
Technically Rambling 