It was just an idea - the only advantage might be that you could easily see the whole frequency spectrum at the same time instead of having to infer it from the lissajous pattern. It probably isn't that useful most of the time. Baudline can show you magnitude with phase as different colors (or real+imaginary), but that can blur together sometimes.
A goniometer is super-useful during synthesis. The recent(-ish) versions of sunvox[1] lets you put a goniometer on any module in the synthesis chain; I love being able to see stuff like the phase of a delay effect.
[1] http://en.wikipedia.org/wiki/Goniometer_%28audio%29