It's generally horrible outside of a last-minute VP grab...I used it as insurance that I wouldn't lose a VP while I had all of my troops committed to taking the last fort I needed to win in one game, but it's pretty much never going to be worth the gems and mage time to cast unless it's a game-ending situation.
If I knew that a Fortress needed a bit of a boost so that reinforcements could arrive, I might use it. In general, I want the enemy to Storm so that I can tear up their army.