I know that the main purpose of GLUT is not to allow the programmer to produce complex GUI but I think that it is time to improve and to add new features.
Many programmers is using this utility to produce cross-plataform apps and I think that we should change the role playing by GLUT.
Not just for testing or for simple apps !!
GLUT is just an API written on top of OpenGL, and is not a part of OpenGL. So the suggestion should maybe be posted to the author(s) of GLUT instead 
Anyways, more functionality usually means a more complex API. And a more complex API will break one idea behind GLUT. GLUT serves it’s purpose very well. It creates a window for rendering in just a few lines of code, which takes a minute or two to code. And even though it’s simple, there are quite a few complex operations that can be done, considering the amount of code needed. It’s possible to write outstanding games using GLUT as it is today. And as you said, we use it to produce cross platform code. This is VERY important. You only have to write ONE file of code, and it usually work on ANY platform. Perfect for demonstrating new techiques.