Actually, you don't need HDMI for watching TV.
The likelihood is that TV has a built-in tuner, meaning with a decent antenna, you can pick up all the local networks in HD over-the-air for free. Even if you get cable or sattelite with HD programming, the likelihood is that you'll be using component for watching TV.
Now, if you're going to get a PS3 or an upconvering DVD player (this plays DVDs and upconverts them to a HD resolution using magick and sorcery) - you'll need a HDMI cable.
Also, what's kind of cool (and I do it), is using your video card's dual display to hook up your PC to your TV and your monitor. I can watch full screen episodes of Battlestar Galactica on the TV while chatting on the PC.
It's neat.