That's the question Wired's "Gadget Lab" is asking, as it gives a quick review of the new Apple iMac's 27-inch monitor. That's only a few inches shy of what you get from an Apple TV nowadays, so maybe having the computer as well as the screen, makes more sense? From their assessment:
The iMac’s killer feature: The screen can be used as a display for another computer using an optional DisplayPort adapter (DisplayPort is a two-way interface). This means that, a few years down the line when you have convinced yourself your computer is too slow, you’ll still have a great TV.