I think "leaking" demos can be way for an artists to test out the material on the listeners. I'm NOT suggesting Mika has done this for one moment, and I'm totally against making illegal mp3s and such. But it's such a grey area in the music industry.
For example, it 'okay' to download a video of, say, a performance of Holy Johnny, because that song is now being performed, though still unreleased??
At the end of the day if Mika has spoken out about this we should all respect his wishes. We can't un-listen what we've heard though unfortunately.