I think it's mostly a usability tradeoff. If you can use the same data type for calendar appointments and high-precision performance logging, then you save your users the necessity of choosing between different types. Bonus points for being able to get away with a single nanosecond-based Duration type that has straightforward arithmetic with nanosecond-based timestamps.

And if you're deep enough to worry about the cost of those extra bits, you probably want to have your own date type anyway, tuned for your exact problem.