The rehearsals are VERY important. I also changed my mind in the past during the semis. Some songs really surprise when performed live... there's this WOW effect which makes you like the whole package (while you didn't like the song alone much before) - and, on the other hand, you will get disappointed by some songs because they are terrible live, not only vocally, but bad staging, bad clothes, bad anything...
I also THINK that after the jury (semi) finals or even before some results will leak amongst the jurors which mabye affect their votes or even the odds, I don't know, it's just something I wouldn't rule out. So the public vote is still the most important but the younger the audience will be in the future the more the odds will influence them too. At the present day there is still a certain amount of viewers who aren't reading the odds or the internet I think, so the public vote is the most honest by now. But it will change in the future. Especially since they (broadcasters, committee) are already trying to influence the public, e.g. by determing the running order etc.