If you're sending humans, that's another month or two of the most expensive supplies in the history of human endeavor. It's not exactly a detail that we can currently afford to wave away.

(And I do mean afford. If we had better propulsion tech, like, oh, say, anything nuclear, we wouldn't have to worry so much about such details. But if we're going to hobble ourselves by rejecting that out of hand, we can't afford much mass.)