HDMI inputs are like $100 bills: you can never have enough.
One usually goes to the cable box/DVR/TiVo; another to your Blu-ray player. And it's not unthinkable that you'd also have a game console, Roku box, and Apple TV. (What, just me?)
If your HDTV has only two or three HDMI ports, you're looking at a lot of cable-swapping. What a hassle.