When I use a grid type screen I use the grid lines as a sharpness reference. For macro work and for using zoom lenses or other slow lenses I find it much easier to focus with a grid type screen than with any combination of microprism and split image. If I know I will want to change focusing screens "in the field" I will use a camera like a Canon F=1 or Nikon F2 which will allow the screens to be changed easily and without any special tools. I might carry two bodies with different screens. Before I had different systems I has Nikon E (for the FE) screens transplanted into two Konica SLR bodies, FT-1 and T2. It really helped with macro shooting. I also like to use grid screens in medium format cameras where the lenses are typically slower. If I am shooting at higher magnification, like 2-5X then I will use a plain matte screen.