Eh, they're fine for when you want files that people can recognize, and aren't huge.

You pass someone a bunch of flac, they don't knot what it is and you waste time explaining it. You pass them mp3s, they happily go on their way.

That's the only real reason to use it since there's better options, and any player can play them nowadays. Might still be useful for older cd players that also handle mp3, but that's the only format they handle.