vtkOpenGLTexture change gray image to RGBA

In some time from version 8 to version 9, vtkOpenGLTexture transforms images with 1 or 2 channels to RGBA format. see around line 277:

        // make sure using unsigned char data of color scalars type
        if (this->IsDepthTexture != 1 &&
          (!directColors || inscalars->GetNumberOfComponents() < 3 || resampleNeeded))

Why do this? It costs vtkImageActor a lot of time to render a large gray image.