That's a limitation of memcached. It could change in the future. Other engines may support more details responses to atomic operations.

I think the use is, if engines did support it is
* for writing - could help debugging
* for deleting - one can see which keys actually existed before deletion

I have already started writing the MemcachedEngine and I've just simulated this.
```
		$success = $this->_Memcached->deleteMulti($cacheData);
		
		$return = array();
		foreach (array_keys($data) as $key) {
			$return[$key] = $success;
		}
		return $return;
```
I just felt we shouldn't limit what engines could do based on memcached only.