Have you ever watched a iTunes SD movie (for example) on a big computer screen and then on a 60" TV? Ever wondered why it looked better on the TV although the resolution is bad?
The reason for this is simple. A TV has less resolution to start from, thous less interpolation is needed. And a modern TV utilizes some image processing units to smooth out motions and upscale lower resolution videos - again, less interpolation artifacts and better color.
But using a TV as a computer display is a entirely different story. First, the resolution is too bad to start from. Text will just look awful on 1080p on 30 inch and up. Second, color fidelity isn't even close to a good computer display, even if the TV features an IPS panel. And last but not least it doesn't look as good and isn't as convenient as a computer display.
I have the highest end LG TV from 2010, which features full-LED backlighting with local dimming, an awesome IPS panel and pretty much the best color in any TV. It even features 480 hz refresh ration (nothing that gives you any advantage in reality), but I would never consider using it as a computer display. It's just not built for this usage. And again, I would never watch a full movie on my 24" LED Cinema Display, although it's great, every movie simply looks better on my TV.