Putting extra charge counters on a Rocket Launcher or Particle Cannon is a BIG DEAL. A 1-cost card that represents 8 damage? Sign me up. Of course, you'll use it on Groot if you have to Groot, but it doesn't do quite as much there.
Consider this an auto-include if you've got Rocket and Groot as heroes at the same table, and you probably play it with one hero solo and the other as an ally too. This is a fun twist on the Team-Up cards we've seen before that it has a more hero-specific effect, and having options is always great.