Kanzenshuu

Power levels establish tension and drama. People who care about them (well, people who care about them in a narrative) don't care about the big numbers or the fancy explosions. If you have character A who's so much above character B, who's the main character, you're gonna be left wondering how in the hell character B, the character we're supposed to care and root for, is going to escape the situation or overcome the odds. It makes us emotionally invested.

If character B doesn't escape the situation in a believable way that's consistent with previous events, then that emotional investment is gone. It was pointless tension, pointless drama made just to suck in the viewer. It has no critical value whatsoever. The audience is left believing that the author can just create whatever scenarios he wants and what happens to the characters is decided by whatever the author wants to happen, regardless of the events that happened in the story. Which, in fairness, is what happens, but the audience wants to be fooled. The audience wants to know that the world they're following has rules. That the world they're invested in isn't going to bend to external factors that are irrelevant to them.

An author can do whatever he wants with the characters, that's not false. But the author should also have the responsibility to make sure it fits in cohesively with the other events in the narrative he has created.