Feels like there is never a reason for adding Edge to your diceroll instead of using it retroactively to reroll, but maybe I missed some finer point here?
In the games I play, Edge is usually a lifesaver (Iīve pulled my schaman from certain death by burning Edge five times, no kidding, Iīve spent roughly 15% of the characterīs earned Karma just buying that single point of Edge back time and again). We mostly use the reroll option to get those very important successes on the soak-rolls rather than maxing out hitdices.
But then again, noone in my team has ever been above 3 Edge as far as I can remember. So it might be true that you use your Edge to do other things than just surviving if you have more of it to spend...