Actually, I think a lot of players are too focused on getting a Valiant. Some of the 4* and 3* (with chance to upgrade further) heroes have pretty sick skills. I'm grooming some of these heroes for my pvp squad.
If you look at the players on the top of the arena, their teams are not filled with Valiants either.
Also, 3* heroes are useful to be deployed on expeditions or fused with your main heroes.