As for why you shouldn't be a yes-man. I am an individual, I'm educated, skilled, and have two decades of experience and what I consider to be generally good judgement. I always attempt to give of my best and to do what is right for the best interest of the project and company/organisation. If that means pointing out facts which are wrong, ill-considered or counter to any prevailing group-think, I would do so every time. It might not always make me popular, and it might ruffle a few feathers, but to ignore problems and evade responsibility by silently going with the dictates of those above me or in the group would be a disservice to the project, the wider organisation and to myself. I refuse to be a spineless jellyfish merely to make life slightly easier for myself in the short term. I'd rather go home each day satisfied that I did the best I could despite the circumstances, than leave knowing I did mediocre or downright awful work when I could have done better.
Maybe I wouldn't be a good cultural fit in some places. But everyone in an organisation bears some level of individual responsibility, and I personally would prefer to work with people who can shoulder it, and show they care about their work by arguing their case well, rather than abrogating it and showing that they don't really care at all about what they do so long as they get paid. In my experience the output of the latter is typically mediocre at best, because they don't give a damn.