@dsc123 - How will you use the CC when it can? Play video files saved on your smartphone? That sort of thing?
You can do that now. If you go to your gallery and select a video or picture the chromecast button is on the top. I've also bought a TV show in the play store and played it on the tablet, streamed it to the TV while reading threads here on TST. The best part is that all I have to do is start the content and hit the stream button. The TV switches to chromecast automatically. It couldn't be an simpler.
One new thing would be to play content from the internet--HBOGO, MLB.TV, ESPN3, random videos embedded into websites, etc, from my tablet. You might be able to do some of those through the apps, but if not, now you can via mirroring. I've wanted to play videos from deadspin or whatever and not been able to since they weren't on youtube and it wasn't important enough to go get my computer.
I also skype with family so they can see my babies and it would be nice to mirror that onto the television. I could do that with my laptop and an HDMI but I find it much easier to use my phone camera.
I can't think of many more specific uses, but basically mirroring would mean no restrictions--if you can get it on the phone you can get it on the TV. The only downside would be that mirroring probably takes control of the device. So the functionality within particular apps would still be useful to let you do other things while streaming. But mirroring lets you stream with apps that might not have that functionality built in.
MLB.TV will be big for me because I'm a Yankee fan in DC. In the past I've hooked my wife's iPad or my computer to the TV but that required wires. I'd probably stop paying for HBO and use a friend's HBOGO account.