The original premise is mistaken in trying to make a distinction between ABC and notation. ABC is simply a way of representing the musical information contained in notation in a way a computer can use. In this it is no different from other music-writing programs such as Sibelius, Finale etc and any comparison should be with them, not with notation.

What seems to confuse people is that the code ABC uses to represent this information is ordinary text, whereas with the other writing systems the digital code is unintelligible without the appropriate software. However that is incidental, and in practice I suspect most users of ABC (myself included) use software to turn ABC code into notation rather than read direct from the code. If some people can play straight from the code without first converting it to notation, that should be seen as an additional level of usefulness rather than a reason to find a false dichotomy between ABC and notation.

I prefer ABC over other programs because it is free, and for writing down simple melodies I find it is easier than the others I have tried. Other programs have more complicated entry systems which make it possible to write more complex music but which I find are slower when entering only a single line of melody. If I were writing more complicated scores then I would probably use something different, but I'm not. If I were usually writing more complicated music and was therefore already a proficient user of another program I would probably stick with that rather than use ABC instead, but I'm not. If I needed more control over the visual appearance of the finished score I would use a different program, but I don't.

I also find it handy that I don't need specialist software. I'm not allowed to install music software on my work laptop, but I can still use an ordinary text editor to write ABC and an online converter to turn it into notation or play it back. At a pinch I don't even need a computer, and for jotting down a tune on the back of a beermat ABC takes up less space than writing out a score (besides I can't draw straight lines) It is also useful that ABC has become the de facto standard for sharing folk tunes over the internet (and can be posted on forums like this where .xml files cannot) so there is a vast library of tunes available in this format. However these are all reasons for using ABC rather than a different program. The end result is still notation.