I mentioned the idea that giant robot fighting games were the only kind that mattered (which is something my sister originally said) here.
Why do I bring it up again?
Because the same game's multiplayer gives the opportunity to unleash combiners during boss fights (you can see one example at around two minutes in, and there's several other fights as well):
This is made even better by the fact that the combiners actually seem to be pretty effective, being twenty seconds of invulnerability and massive damage before decombining.
(Find more information on the game's various combiners here, if you're curious. Only really pictures, but they show the combined forms of all four player characters, which actual gameplay doesn't allow.
...Seeing them all in the same place, incidentally, really emphasizes the "unpossible" nature of their rather Getter Robo-esque transformations.)