While in the Transform or Scale mode, you can move or scale along the X,Y and Z axes without actually having to click on the tool. Simply MMB click and drag anywhere within the viewport and move it towards the direction you wish to go to.
If I recall correctly, it was dropped in favor of a uniform scale because the binding of mouse axis to scale axis didn't work well and was difficult to control.
Perhaps it will return in the future if the developers figure out how to make it work satisfactorily.
