It's not always a tradeoff. Several sites stopped serving CSS during the Superbowl (https://twitter.com/jensimmons/status/828415747625992192). At a smaller scale, the same thing happens to some sites that reach the HN front-page.
It depends on exactly the costs are. But in that example, those sites are spending millions on Superbowl ads, and it's probably their highest day of traffic, so it's not "almost no benefit".
