I love it when I have a lot of visitors, because my butler always IMs me the list. Then I can easily refer back to it any time without scrolling back for ages in local chat! When there's only a few visitors, if I want to pay them return visit it can be a pain. Either I must open each of their profile pages in a new tab (annoying, since every link I click after the first wants to load in the same tab instead of opening a new one. I have to do that manually) or scroll back for ages through local chat.
If the "Who Visited?" list were always sent as an IM no matter how many or how few, I would never lose track of it!