Mainly for our own workflow, but also so players have a chance to comment. If we wait until the end to integrate all this, that's going to be excruciating. It's going to be buggy for a while, plus the volume of integration work is uncertain and likely to cause delays for us. But given this is only releasing to people playing the post-release betas, people get to help us out doing what you do in a beta -- making sure everything stays sane.
On the other side, heavy cat is receptive to feedback. However, if players can't see the new art in game until the end of the process, there is no chance for players to be involved in that at all. Seeing images out of context or even in screenshots is simply not the same as seeing them in-game.
In terms of official releases: yes, we're releasing this all at once in a few months as something like avww reloaded. It's just the beta that will see things be in the in progress styles.