Frequently heard, but I suspect the people suggesting it have never done any data input work involving lots of numbers.

I could argue that you don't need a mouse. If you're moving from keyboard to mouse so frequently, why not learn how to do things without the mouse?

I had a job in logistics where I often had to train non-IT professionals to handle data input involving mostly numbers. Most of them had never used the numpad, half didn't know how to move around using tab/shift-tab and didn't know keyboard shortcuts. They would rely heavily on the mouse. When I trained them to use all these, and forced them to use by removing the mouse, their productivity would increase by several hundred percent in some time-consuming data input tasks. The software was Microsoft Dynamics and Excel. Most of them were frustrated at first, but became proficient within a week or so, and would continue to use the keyboard even after reintroducing the mouse.