They've actually addressed this one, and mentioned that they're saving it for full release, but they will be including it.
Sorry to burst your bubble... many times developers promise this or that.
They still have a time table... it could be that decide that multi classing will be done after full game release at some point.
Do not get me wrong I like this game and I am happy with patch 4 content and that development resource has increased a lot and that we get Druids class and other good stuff.
I am not saying they will not include multiclassing at release. I am only saying it is not 100% certain.