This is motivated by the fact that, if one YAML file has a param with a mispelled name, then the default will be loaded. And the user will never know. To avoid unpredictable behavior following this circumstance, I think the best way to go is forbid defaults completely.
Please check out your applications to make sure you are on the safe side.
MR !305 (closed) addressing this issue will be merged soon if you do not signal any inconvenience in the following days.