Unused keys are really lost opportunities for everyone. For developers, one of the main reasons to participate in a bundle is to create exposure, to put the game in the hands of gamers who might not otherwise find the game or buy it at retail. The hope is that some, perchance many, will actually play the game, like it, post reviews, tell friends, generally spread the word, which ultimately provides a promotional boost for the game and the developer, something particularly important for indies.
And, no doubt, not every game appeals to every gamer, though conversely, most every game will have an appeal to some gamers. Keys that were included in a bundle that are left idle represent the opportunity for joy unrealized by some gamer.