A week ago I would have agreed with you. But now I am getting used to this way, and this is really a nice feature when you follow a particular thread. It's only when you open the thread for the first time you want to read it from the very beginning.
But having this as a selectable option would be nice though ...
Perhaps xoops could keep track of the number of times a user has visited a thread (a new xoops feature, perhaps as a selectable option)? If it is the first time then the thread is displayed from the very beginning, otherwise it shows latest post (or the post that was the latest at the time the user last visited the thread) ...?