Right now the last rank of ED does absolutely nothing. I'm reasonably certain that it's because originally the character level was calculated as 20(heroic)+ED level. So to achieve level cap at 25, ED had to cap at 5. It was changed to 20(heroic)+actual epic levels, but caped destiny remained lev5.
There should be some benefit for caping destiny, making it lev6 would mean another fate point or it can give 2action point. Anything would be better than nothing.