This was my initial reason for preferring a Qt-client over the official one, but we had many other things to do first :-)
Since messages can be edited afterwards, and to avoid dupes when reconnecting and history is replayed, the local storage is a SQLITE database. The user can however export that DB as a text file, on demand, using the action "Display / Show Channel Log".
Maybe in a few years we'll have to ask for a start date to avoid exporting many years, if that takes too long; we'll see :)
If you think the format of the text export is similar to konversation logs, that's, hmm, pure coincidence :-)