If you thought your boss would resent you for taking the full benefit and would rank your performance poorly, would you take that risk knowing that you've now got a very small person who is relying on you? You could potentially be limiting bonuses, raises, and promotions for an extra few days off. I'm not saying this is right, but I can imagine a reasonable new father feeling this pressure.

I doubt many people shorten their paid time off simply because they love work so much.