I find it very useful in Chrome, and use it the same way as iBobi; to check the contents of the thread when the title is too vague. When it does come up on threads I'm not intending it to, I just move my cursor to the far left or right of my browser, into some clear space.
The tool tip summaries only appear (in Chrome/IE at least) when the cursor is hovering over a thread title URL.
If Safari is doing a slow fade out instead of a more instant disappear I can understand that could get annoying. Don't know if there's an option to allow users to set their preferences, but if it's an on/off for everyone, then I personally hope they would stay on.
Sorry tim0409 and zdog2x.