Don't forget that Det also increases DPS as well. So not only will you be getting better heals, but you will also be getting stronger DPS. And Piety is only good until you have as much MP as you are comfortable with. If you get too much Peity it's a waste of stats because you don't need that extra MP and MP/3. Spellspeed is viable in any content, it's just not optimal. If you want to build SpSp, go ahead.