Why is a reference plane needed in differential signalling?
More to the point the common edge coupled pairs that everyone uses on PCB layouts are not usually particularly tightly coupled (Think about the geometry, the reference plane is the same order of spacing as the two lines, and the plane has a dielectric in the gap! 

They are a lot better then nothing, but the impedances are mainly defined by the coupling to and from the reference plane rather then the coupling between the lines, and line balance is mainly a common mode impedance thing, the reference plane helps here.

73 Dan.