Encapsulation
If the ivar is private, the other parts of the program can’t get at it as easily. With a declared property, the clever people can access and mutate quite easily via the accessors.
Performance
Yes, this can make a difference in some cases. Some programs have constraints where they can not use any objc messaging in certain parts of the program (think realtime). In other cases, you may want to access it directly for speed. In other cases, it’s because objc messaging acts as an optimization firewall. Finally, it can reduce your reference count operations and minimize peak memory usage (if done correctly).
Nontrivial Types
Example: If you have a C++ type, direct access is just the better approach sometimes. The type may not be copyable, or it may not be trivial to copy.
Multithreading
Many of your ivars are codependent. You must ensure your data integrity in multithreaded context. Thus, you may favor direct access to multiple members in critical sections. If you stick with accessors for codependent data, your locks must typically be reentrant and you will often end up making many more acquisitions (significantly more at times).
Program Correctness
Since the subclasses can override any method, you may eventually see there is a semantic difference between writing to the interface versus managing your state appropriately. Direct access for program correctness is especially common in partially constructed states — in your initializers and in dealloc, it’s best to use direct access. You may also find this common in the implementations of an accessor, a convenience constructor, copy, mutableCopy, and archiving/serialization implementations.
It’s also more frequent as one moves from the everything has a public readwrite accessor mindset to one which hides its implementation details/data well. Sometimes you need to correctly step around side effects a subclass’ override may introduce in order to do the right thing.
Binary Size
Declaring everything readwrite by default usually results in many accessor methods you never need, when you consider your program’s execution for a moment. So it will add some fat to your program and load times as well.
Minimizes Complexity
In some cases, it’s just completely unnecessary to add+type+maintain all that extra scaffolding for a simple variable such as a private bool that is written in one method and read in another.
That’s not at all to say using properties or accessors is bad – each has important benefits and restrictions. Like many OO languages and approaches to design, you should also favor accessors with appropriate visibility in ObjC. There will be times you need to deviate. For that reason, I think it’s often best to restrict direct accesses to the implementation which declares the ivar (e.g. declare it @private).
re Edit 1:
Most of us have memorized how to call a hidden accessor dynamically (as long as we know the name…). Meanwhile, most of us have not memorized how to properly access ivars which aren’t visible (beyond KVC). The class continuation helps, but it does introduce vulnerabilities.
This workaround’s obvious:
if ([obj respondsToSelector:(@selector(setName:)])
[(id)obj setName:@"Al Paca"];
Now try it with an ivar only, and without KVC.