In this post( http://android-developers.blogspot.com/2012/01/holo-everywhere.html?m=0 ) by Adam Powell, a Google engineer, it is explained that all Android 4.x devices will have to have the Holo UI and widget files on board to have access to the Android market.
Whoa! Now, everyone rein in a little, no where in the post did he say that they had to be used in the manufacturer's UI. That post can be whittled down to this.
The manufacturer can and will still be able to skin the crap out of the Android UI. However, the Holo widgets (buttons, color styles, and other UI elements) have to be available on the device for developers to use.
This means quite a lot in the long run, if your HTC or Samsung the developer now has direct and easy access to the stock Android UI styles thus encouraging said developer to use those styles,instead of the manufacturer's, so his/her app will be consistent across all devices.
Will this put an end to manufacturer skins? No. Is it a move by Google to start to unify the look of Android? Yes.