I had the same thought when I was upgrading her quickly before this tournament… but the reality turned out to be quite different.
Bear is a necessary hero in the fight against Trees (one green slot out).
Only Faiez can prevent Joku from “turning off” buffs (with Bear). He additionally heals and protects against all (not only def down) negative effects (second green slot out).
Belith… what can I say? There are too many Faiez and Bears buried in the Trees for me to give her up 
Of course I could use a Tyrum instead of one of the trees… but then I would fight with only one healer - too risky for me (third green slot out).
Fighting with four heroes from one element is also too risky for me.
So there is no room for Zarel in my attacking team… Especially since her doesn’t “accelerate” either Tree or Faiez (my other key characters).
…and reduction is not needed if you have Faiez protection
However, everything has its pros and cons - everyone chooses what is most important to him 