My reasoning is the same the first time i suggested it during CBT2. Alliance are a place where you interact with your members, the chat clearing whenever you relog, greatly reduces how you interact with your members. Atleast let the chat stay for 24 hours, or delete the messages once the 24 hours passes from the original time of post.
Another thing, put time stamp on each messages, you can even limit the amount an alliance member messages to once every 30 seconds.