Actually, it introduces a new possibility: it decouples the YAML config from the DIC parameters, which means that extensions can make their config more semantic (using a nested array to group related settings), can validate the config provided for the extension and can provide BC for older formats when it gets refactored. Btw, these are the reason why Symfony2 bundles have a system for semantic configuration instead of relying on the user to set DIC parameters directly.