Mostly, I use the CA glue to hold things in place while I do the fiberglass work which is where most of the repair strength comes from. It's very handy for holding broken bits in alignment before the actual repair process begins. It is sorta reversible if I screw up, because a soak with acetone for a few seconds will release the bond so the parts can be realigned. You just need to wait for 5-10 minutes before reglueing to allow the acetone to evaporate.
For the thin cracks in the outside surface where it's difficult to build up much fiberglass, the glue does go a long way toward keeping the crack from reappearing after the filler is put on, but the main purpose is to hold the broken bits in the proper position. Using Duraglass filler is important because it has more structural strength than plain Bondo type fillers because of the glass fibers in it.
