It's not unforgivable it's just the story. Gohan isn't underutilized if he's actively involved in a story line and at the moment he is the focus of it.
And I'm saying that the story is poorly constructed at the moment. Gohan isn't really involved in the story. He's had the grand total of 1 episode dedicated to him and it's spent demonstrating how much his fighting skill has degenerated in a show that completely revolves around fighting.
Gohan ain't Goku 2.0. They're really driving that home. Basically, "Chi-Chi was right and Goku was wrong."
Also- RoF was also about Goku learning his lesson about giving bad guys benefit of the doubt. Vegeta also suffered from his own set back - cocky showboating. Weird to say it's lousy writing when Whis made it very clear what their weaknesses were from the beginning (ego and carelessness) and because of them the Earth was initially destroyed.
Again, this doesn't make sense. You could easily write into the story that Gohan trains and does his family/scholar stuff. If normal humans can do that, I'm sure he can.
ROF wasn't really about Goku learning his lesson. It was about getting a cheap shot on Frieza because Toriyama can't stand any other character getting the kill other than Goku. If you want to demonstrate Goku learning his lesson, that is simply one of the worst ways to do it.
Vegeta's set back wasn't showboating... That was Goku's :S
Vegeta's set back was that he always lived in Goku's shadow because he didn't learn to relax. The movie could've easily been about Vegeta learning his lesson which would've made a lot more sense considering he spent the majority of the time resting and not really caring about the fight.
Really surprised anyone can defend ROF's ending. It is easily the worst thing about it. It had good fight scenes and an interesting premise but it was ultimately let down by a very cheap ending.