It's not a Chromecast specific icon, it's intended to be used to select which output device to use from a list. This is mentioned in the FAQ where it's referred to as the "Cast" icon.
I write a bunch of apps right now that have the ability to show video has one of the many functions of the app. Having to put an icon like this on every screen would take up a lot of screen real estate and I think confuse a lot of our users.