It can be convenient sometimes to look at the glyphs when visualizing a DTI and to overlay the glyphs to the Fractional Anisotropy or the Mean Diffusivity.
The Volume module in Slicer allows the user to select the scalar mode that is displayed when a diffusion tensor image is selected (ColorOrientation, FractionalAnisotropy,Trace,...). However, the glyphs representing the tensors can only be displayed in the 2D views if the selected scalar mode is "ColorOrientation". The only way of visualizing the glyphs with the FA or the MD is to compute it, select the DTI as the foreground (or background) image, select the FA/MD as the background (or foreground) image, and then move the slider that selects the transparency of each layer such that the FA/MD image is visible. This way, the glyphs are visible on top of the FA/MD, and not only on top of the ColorFA. This workaround is not very complicated, but one may wonder why it is not allowed to directly display the glyphs on top of the FA/MD that is directly visible from the tensor image by changing the scalar mode to the appropriate value.