Good mastering requires accurate monitors, as well. Every stage gets tested in various types of speakers and playback devices and formats (so, the engineer checks it out in their car on the way to the studio, on their iPod, on their laptop, etc., as does the mastering engineer). There is no stage where the engineer (recording or mastering) doesn't want an accurate monitoring setup, however. And, there is no stage where checking the mix against various listening environments is a bad idea. High dollar recordings are listened to hundreds of times by at least a half dozen people on the way to being shipped to listeners.
I'm probably commenting more on my experience, which does not extend to actual hits. But I am called on to emulate the sound. It has been a struggle to get the bass right, often I find if I mix to have perceptually a lot of bass on, let's say small computer speakers, it's easy to have blown out bass on a system with a sub.
The bottom line is that virtually all playback systems that exist in the real world are nothing remotely like an accurate studio monitor. The engineer is always concerned with how the mix translates to different systems. In this process accurate monitoring is profoundly helpful, but that doesn't mean the engineer expects any listeners to have such a system.
Bass enhanced headphones like Beats have become yet another listening environment to be considered.