The main reason for a filet is so the glass will have contact with the joint and not be suspended in air. On outside joints it's not necessary. On inside joints it's necessary to run a filet because the glass cloth will suspend itself over the tight crease of the joint. A filet by itself does have strength, but the glass adds tremendous shear strength. Put a piece of glass or glass tape in a vice and try to rip it. It's impossible if its anything over 4 oz. So, with glass over a filet, you are guaranteed that the joint can never separate. When I do a S & G, I run my filets on the inside joints and then before the epoxy putty hardens, I run my glass tape right into the soft filet and wet it out with a brush and some unthickened epoxy.