Where I find a mouse useful is when I want "that loop which I suspect is misbehaving. That one which adds a new member to all arrays". There a quick flick of the mouse-wheel, and then fine positioning of the mouse cursor I find gets me to my location faster.
Even when the loop is on the screen, maybe 35 lines up, I have to either do a binary-search style jump with the keyboard, or with the mouse just click on it.
Also, looking for a particular word is easy when there is only one occurrence of it, or you want the next occurrence. A mouse might still be useful when there are many occurrences of a variable and you want (for example) one somewhere near the end.
Perhaps vimmers tend to keep better track of where everything is to make it easier to "hard jump" to, while mousers tend to keep a more ad-hoc overall view, so they know approximately where to scroll to?