I'm really not sure why everyone keeps crying about ass pulls like senjutsu is something new in the Naruto universe. It really isn't. Oro is a connoisseur of a great deal of jutsu knowledge, so its no surprise that he ran so many experiments and tried to find ways to manipulate... well, just about everything. Sasuke wasn't being a complete dunce when he followed him those years ago leaving Konoha. He didn't select his team at random. He wasn't a scientist and didn't know the specifics, but it seemed he was at least aware of senjutsu and nature energy, and that Jugo was connected to it because of his tribe. So to be fair, he had used senjutsu in his techniques before we even got to shippuden when he had the curse mark, courtesy of Oro and Jugo.
Secondly, since we all know what its going to get down between anyway now that we're wrapping this up, why wouldn't Sasuke acquire some means to fight back? As much as some of you may not like him, we all know the characters Kishi wanted to center parts of the story around and Sasuke carried a big part of that. I think people are ignoring the perfectly logical explanations that have already been given before this, and if I may also add, this doesn't even really count as a "power up" unless he somehow permanently absorbed Jugo, which I see as unlikely. I'm pretty sure this mode will have no additional purpose other than to fight against the Jubi tree...thing.
Lastly... people have an issue with Jugo transferring nature energy as a senjutsu form, when he already previously transferred parts of his body to Sasuke to keep him from dying from Bee's attack? Wtf?