Feature request from ihor-sviziev, posted on GitHub Oct 21, 2014
From Magento 1.x we have a lot controllers that use _addContent. This method is fires after running loadLayout. As result we haven't ability to use action directive from layout files to use some manipulations, as example change template file for block (described there).
I propose mark this method as deprecated and replace same places to layout updates.
List of this places:
https://github.com/magento/magento2/search?q=_addContent