@ Bakuda: I'm sorry for taking this a little offtopic.
I will start with the less radical part of how NDAs hardly give you any advantage:
- First of all, nobody forces you to make all your content public. Just say "hey, I'm making this <short description> game, contact me if you're interested", you don't need an nda to do that.
- Of course the nda is mainly about that person then revealing all of the content. Fortunately, our brains are quite capable of evaluating whom we can trust. If the other one is someone trustworthy, you can simply ask them to please not reveal anything and they most likely won't. If on the other hand the other one is someone out to screw you, they will do so with or without an nda, there's always a way.
- The above is even more true considering that an nda is nothing but a piece of paper with a little bit of ink applied if you're not willing or able to enforce it. If someone actually breaks the nda, it will cost time, energy and money to actually hold them accountable for that. Even if you are willing to move time and energy from the gamedevelopment to the courtroom, being an indie-team, you most likely won't have the funding to pay a lawyer.
- "But what if someone steals my idea?". That is certainly the most-asked question in this context. And even though it's been said countless times, let's repeat once again: ideas are cheap. They're a dime a dozen and anyone working in gamedevelopment has more ideas in their head than they'll ever be able to produce. What makes a good game is the execution, not the initial idea. The only reason someone in indie would prefer to steal your idea instead of working together or making one of his own, is that they don't have any. But if that is the case, they'll hardly be capable of making that idea into a game any better or faster than you.
- The other main reason for NDAs is marketing. The trick with mystery-marketing however is that, while no one knows what the answer is, everyone knows that there is a 'mystery' (in this case, the content). The big industry can pull that off pretty easily, they can just push content into the media until everybody is sure to have heard about that at some point. Indie teams on the other hand do not have the budget for that and mainly rely on word of mouth advertisement, and it's just really hard to get people to say "I've seen this cool upcoming game. I've never heard of the guys who make it and I don't really know what it's about, but you should definitely check it out." Maybe there are examples where it worked and it would be great to hear of them, but most of the time, indie-gaming is still too small and at least I wouldn't know of any cases where that really worked on a large scale.
Now for the real deal, why is it actually bad for development:
- Methods and standards have evolved in game design and continue to do so, but in it's core, it is a creative process. And like any creative process, it relies heavily on the dynamic within the team and the motivation of the individual members (even if money keeps them working, motivation is what produces quality). NDAs damage these dynamics by introducing yourself with two basic premises: Even though you might make a 180°-turn from outside the team to inside, the very first connection between the team members on which anything else is built will be distrust. Secondly, even before the new guy is joining, you define an unhealthy hierarchy: You are the boss and you alone decide the rules under which one might work with you. While I certainly don't say there shouldn't be any hierarchy at all, the role of the team leader should be a supporting one, not the bad boss that want's to control everything.
- While there might not be many that are higly opposed against ndas in indie-teams, the simple fact that they have to make quite an effort to only view the most basic content of the game and thus even consider joining the project is a huge inconvenience for many people when looking for a team. The result is that quite a few of them simply won't and stick with a project that tell's them what they're up to from the beginning instead, resulting in fewer that are even contacting you. Those that do contact you are then likely to contact the other teams with ndas as well, resulting in them having lot's of options which project to join. All in all, it will result in you having a much harder time finding members than you would have had otherwise.
- As Katz already said, this will also take away the exchange with other developers and the community. While many might do well enough without input from other developers, a quick glance at the gamedesign forum shows that most of the people that ask for advice there get tons of ideas they wouldn't have ever thought about on their own. Even more important might be that this doesn't apply to design-questions only, but also to tech-questions. So especially if someone on the team is inexperienced or trying out something new, they won't be able to ask "I'm trying to <do this>, how do I accomplish that?", making work much harder for them and lowering the quality of the product.
- Finally, a game idea evolves and changes heavily during production. While it's always good to have a design document, an nda requires you to have a very detailed one so that as many of your ideas as possible are covered by the nda, and then reinforces the notion that those ideas are "worth protecting". This leads to a very static design process, where many deferring roads remain unexplored and lots of potential goes unused.
TL;DR: Lot's of reasons why one person believes that NDAs are bad for indie teams.