For those who are sexually active or hope to be, a condom is essential. For those who are nuns, eunuchs, Nikola Tesla, Hans Christian Anderson, Isaac Newton, or play tabletop role-playing games, they are merely strongly advised. In the words of some kid I was talking to one time, "it's better to have one and not need one than need one and not have one". This is true not only because of the (albeit distant, for most) possibility of unexpected sexual intimacy, but because of the other things condoms are useful for. They can be used in lieu of a plastic bag for virtually any situation, for example pet waste disposal, and in a survival situation can hold up to a liter of water with ease. Plus, even if you don't need one for its intended purpose, it's possible that someone else, at a social gathering, for example, will, and isn't this planet a bit crowded already?
But, didn't the Greeks get 'out' of the Trojan and destroy the place?