GLEW isn't 'required', as you put it. You could use glcorearb.h header, or anything like that. However, if you link with some function - it must exist on target platform, or your program wouldn't launch. GLEW and others are exactly to handle that - you're not linking with GL functions directly, but instead getting function pointers after initialization phase. It allows you to check at runtime which extensions are present and which functions may be used.
The only thing it helps with portability is getting function pointers - it may be wglGetProcAddress/glxGetProcAddress and it's analog for apple OSes. So no, it's not a case. However, the variety of available GL extensions is.
GLEW requires no touch points with your preferred GL initialization library - just call glewInit after creating GL context.