Having a 'custom noise done in GLSL code' makes your shader deterministic and more portable.
Agreed, and this is what this code does well, right now. The fact that it also manages with GLSL 1.20 amazes me, because that means a direct portability across the board: OpenGL, WebGL, OGL ES.

For me, the ideal situation would be to have a choice between a "shader noise", where you had complete control and perfect repeatability across platforms, and a ten times faster "hardware noise" that came at about the same cost as a texture lookup, to use for shaders with lots of noise components. In software shading, it is both common and useful to have dozens of noise components in a shader.

(Anybody from the Khronos GLSL workgroup reading this?)