I've seen such a feature in image reference software (not sure if I'm allowed to name any examples since advertising is prohibited). Basically it allows you to zoom quickly and precisely without using your mouse wheel. Technically it probably just takes the difference between p1, the mouse cursor position at the time of RMB click (on which the holding begins), and p2, the current mouse cursor position while RMB is being held down, maybe relative to canvas scale, and adjusts the zoom level based on that (zooming in for positive p2.x-p1.x, zooming out for negative p2.x-p1.x; by the way, in the software that I mentioned zooming amount seems to be based entirely on X-axis difference, which I suspect is because if you take 2D difference it would result into jerky zoom-in-much/zoom-out-much behaviour while the cursor stays around p2.x-p1.x=0 and |p2.y-p1.y| is sufficiently large).
The reason I think it would be useful, besides that it looks cool and allows finer navigation control, is at least the ability to reduce the load on the mouse wheel: when I'm working on something in Paint.NET, I'm zooming in and out all the time, wearing out the wheel a lot. The RMB/LMB-click-to-zoom feature that's already there kind of allows to move that load from the wheel to the buttons, but this way I have to switch tools every time I need a rezoom and I still have to click a lot and wear out the buttons. Just moving the mouse around the table aside from being the least noisy seems the least wearing the mouse out. And the existing temporary navigation tool with Spacebar-hold seems (at least to me) to be the perfect place for such a feature, also considering that holding the RMB (while in temporary navigation mode) seems to have no functionality bound to it at the moment.
Here's what it could look like: