Classical music has always been cheap. Or at least, you could usually find a cheap recording of a piece, instead of going for a big-name orchestra.

Most people don't care, sure. But there have been examples of giving away free books to cause excitement and arouse interest. Classical music is not hugely popular nowadays, but this might help more people hear it and enjoy it, getting them to the place where they do want several recordings of the same piece.

Then again, this is all hypothetical. I doubt anyone can predict what effect this actually has.

Edit: One last point. You can hear almost any piece nowadays with a quick Youtube search, and that includes any quality recording you want. For people just looking to hear a piece quickly, I doubt Museopen will have anywhere near the effect that Youtube already has.