Having played your game for a while now, if I had to guess the design purpose behind the Bush Whacker is periods of burst dps to get past difficult sections, and more importantly, bosses. However, one of your other design choices, xp, runs COUNTER to your design choice of the Bush Whacker. Even if its more powerful to put him in, it still feels bad.

I believe a good fix would be to either:

a. Have a new passive on one of the Bush Whacker's later abilities that lets him earn xp even when benched.

b. Instead of this being on a passive ability, have a specific idol you can "buy" with the above ability once the idol shop is opened.