Actually, this is the main reason that my ABC Tune Finder site only deals with
ABC so far.  It's the only online music format I've found that 1) has enough music
available to be worth the effort of programming, and 2) can be parsed easily
without a huge effort.  I've looked around for other usable formats.  I've considered
adding Lilypond and Rosegarden to the list, because they both have published
syntaxes that don't look to horrible.  But so far, neither seems to have enough
music online to be worth the effort (more than for ABC) to write the parser. And
getting converters to all the formats that I currently deliver is also questionable.
Similarly, neither of the XML music encodings seems yet to be in use enough
to make the effort worthwhile.

Of course, this is just a comment from one programmer writing one package
that a few people find useful.  Others' mileage could vary a lot.  And some people
do seem to find the fancy proprietary encodings worthwhile for their own uses.
They just aren't very useful if you want to share your music with others or write
your own software to do something with the music.