In some cases this is less helpful than you would think. I just bought a new receiver to replace one that didn't have HDMI.
In my old setup, my cable box and my BD player were connected to the TV with separate HDMI cables and the audio was sent to the receiver with optical or coax cables. The TV would talk to the BD player over the HDMI cable but that was it. Now, the cable box is still conneceted directly to the TV by one HDMI cable and the BD player is looped throught the receiver to the TV with a second set of HDMI cables.
The problem is what happens now when you just want to watch TV, you turn on the cable box and the TV, and the receiver detects the TV has come on and turns itself on. When the receiver comes on it switchs itself to the TV as a source and disables the audio coming from the TV. You either have to walk over and manually turn off the receiver to make the audio come from the TV, or switch the source on the receiver to the cable box in order to hear anything.
In theory CEC is a great feature, but it can get very confusing and doesn't always produce the result you want. I can turn off CEC on the receiver, but then it won't talk to the BD player, which is very helpful. It would be great if you could disable specific devices from talking to each other.