Imho this is by design and not a bug - Shift before MMB just selects navigation mode: Orbit or Pan.
Are you proposing a new feature - possibility to change (with Shift) the navigation mode on the fly while holding MMB?

50% of the time I hit shift 1 microsecond sooner than the middle mouse button as I try to work fast. I have to slow down to make sure I am compliant with the limitation and press shift first.

I am proposing an improved behavior: The mode selection is to be chosen only once the mouse starts to move. If the pointer hasn't moved then shift should still be available to switch modes.

@Eric (DragonSkunk) - I am proposing an improved behavior: The mode selection is to be chosen only once the mouse starts to move

The problem is that this isn't different modes of one operation, but two entirely different things. If you look in Preferences / Keymap, then 3D View / 3D View (Global) you will see two different entries for Rotate View and Pan View, each being for the two different operators view3d.rotate and view3d.move. Once one starts it can't just switch to the other. And it would just feel laggy if you had to wait before an operator was chosen.

This is a much needed fix. So a simpler solution:

Disable mouse rotation, this option will cause the middle mouse button to only pan without using shift.
Enable mouse rotation and it returns to the default behavior.

This would limit view rotations to the numpad. Useful for the grease pencil to make 2D animations or drawing. Making bone shapes or working on paths, modelling bezier curves, or any procedure that requires only a 2D view.