Well, at first glance your statement appears to be valid.
Yet upon further inspection you should notice a subtle difference between

[array objectAtIndex:0];
[array firstObject];

Try to run those two on an empty array.
Result: The former method will throw an exception whereas the latter (category) method does not, returning nil instead. Just like Apple’s “[array lastObject]” uses to do it.

Summing up: [array objectAtIndex:0] != [array firstObject] and by that in no way “pointless”.

Doing all that extract/unneeded work… to create your own firstObject is like saying:
The first book on that shelf is the first book.

Your code is an excellent example of “categories”… but used in a pointless way.

“Category” is a VERY powerful thing. Use it as such.