It's not about building consensus together, it's about sneaking your "consensus" into the group by the means of divide-and-conquire. It's very hard to build a solid alternative consensus (or a defense strategy) if all the opposing points have been voiced independently, and whatever one you can think of ends up with "oh, we discussed this with the other party, and it wouldn't work".
Please respect your team and don't use nemawashi. If you are on the other side, learn to recognize it and call it out.
TL;DR: nemawashi considered harmful