Is it possible to allow the caching to work without modifying the Pipeline addon? They are separate so ideally you shouldn't have to modify Pipeline. What if mojito itself also passes undefined in the instance object? Also there might be a very slight performance impact by doing this in addition to making the code less direct.

Either way, the array of property here has no effect other than to serve as a truthy value for the 'overwrite' optional argument. Check the Y.mix documentation (http://yuilibrary.com/yui/docs/api/classes/YUI.html#method_mix). The result is that command.instance will be exactly the same as task.specs. This seems like what was intended:
```js
Y.mix(command.instance, task.specs, true, ['base', 'type', 'action', 'config']);
```