Do you take GT as canon?

[JulieYBM]

1. To play along with Gokû?
2/3. Lazy design work?/Wasn't he out of ki?
4. Well, positive energy v. minus energy is typically a common trait one will see in these sorts of stories.

Sure, I'd have preferred Gogeta to finish him off, but...well, Toei's not quite one for integrity in their storytelling.

[Conan the SSJ]

No, because the Spirit Bomb is the most overused finishing attack in the whole series.

Overused? In context of the movies, that's hard to argue with. Goku used it in 4 out of the first 7 DBZ movies (twice in one), though those are all instances of being outside the manga and anime. In the series itself, he only ever used it 5 times; four times in battle (Vegeta, Freeza, Kid Buu, Super Yi Xing Long) and once in a training session. And in battle, it only ever worked twice. I wouldn't call the Genki-dama/Spirit Bomb an overused concept, at least not with the manga and/or anime.