Jason Snell's thoughs on the iMac with Retian 5k Display

Perhaps it’s the most practical solution:

Unfortunately, this display is an iMac—and not an external display. But it makes sense, in a way. Getting this much video data out of a computer and into a monitor can be hard. What makes it easier is if you just connect the computer directly to the display and create a single product that’s got the computer and display integrated. You know, like a Retina iMac. And so here we are… at least for now.