Not the complete game will be released for alpha/beta testing but rather just a first chapter/ a first starting area.
As to whether it ruins the experience - it depends on your individual playing style. When I played D:OS alpha, I mostly only played around with the systems (combat, stealing etc) and skipped all the dialogue, so that I could fully enjoy the game when it was released. However, I think a few people got bored of the first starting town after the alpha.
