I have a bunch of open source classes on Github, so occasionally some kind people send me bug fixes. I'm grateful for that, and it's one of the reasons why I make the code available. Some people also add new features, which is also very cool.
However, in many cases, they add new features by changing my class. And that always makes that little object-orientation alarm clock in my head go off: One of the reasons why subclasses are available in OOP is to let people use and modify library code without having to copy and paste or touch the original. If you change the class someone else writes, and that person decides that this change, for whatever reason, won't be merged into the main code base, you will have to use FileMerge each time a new version is released, to apply your changes to the new version.
However, if you write a subclass that adds the features, you just drop in the updated base class, and the magic of inheritance will take care of doing the "merge" for you.
Mind you, I'm not complaining here. I've received some great submissions for some of my classes, from people like Todd Ransom, David Sinclair, David Rozga, and many others I've probably forgotten. Not to mention that, after I've stared at the same code for weeks, I just don't see that missing [super dealloc] call anymore, and I'm grateful for the people who e-mail me and tell me. But I sometimes have plans for a certain class, and some changes some people suggest would collide with those. Or a certain change would break my or other users' applications because it changes the behaviour of the class, and I have to turn it down.
If you write a subclass or a category that applies your changes, not only is your code completely self-contained and you don't have to merge text files, it also makes it very easy for me to look at the code and see only your changes, without having to fire up diff myself, which may not be possible in an internet cafe anyway. And if I want to integrate it into the "official" version, I can easily copy and paste the stuff over. I'll have to merge anyway, because I may also have made changes in the meantime.
Now, I won't turn down a feature because it didn't come as a subclass. I'm just saying that you could have a Shiny Happy Object-Oriented Feeling(tm), make it easy on yourself to get future updates until I get around to adding your modifications, and make it easier for me to check out your changes on the road.