Why does NSOperation disable automatic key-value observing?

Question

When working with a custom NSOperation subclass I noticed that the automatic key-value observing is disabled by the [NSOperation automaticallyNotifiesObserversForKey] class method (which returns NO at least for some key paths). Because of that the code inside of NSOperation subclasses is littered by manual calls to willChangeValueForKey: and didChange…, as visible in many code samples on the web.

Why does NSOperation do that? With automatic KVO support people could simply declare properties for the operation lifecycle flags (isExecuting etc.) and trigger the KVO events through the accessors, ie. the following code:

[self willChangeValueForKey:@"isExecuting"];
executing = NO;
[self didChangeValueForKey:@"isExecuting"];
[self willChangeValueForKey:@"isFinished"];
finished = YES;
[self didChangeValueForKey:@"isFinished"];

…could be replaced by this:

[self setIsExecuting:NO];
[self setIsFinished:YES];

Is there a catch somewhere? I just overrode the automaticallyNotifiesObserversForKey to return YES and things seem to work fine.