Every true life story of explorers of new lands I've ever read tells of them keeping a journal. Often the story of their exploration is taken directly from that journal. I've never played any game that I didn't keep a notebook on my comp desk to take notes in. Stopping to take notes while I go doesn't reduce my immersion, it adds to it.
If the only purpose of an in game notepad is to replace my notebook, I wouldn't really support spending dev time to add that functionality.