I recently wrote about why I think every blockchain needs a constitution. In the comments there were two broad misconceptions that need to be addressed.
The purpose of the constitution is to define things that are essentially undefinable in code. This means that the goal of the constitution is to define things at the very high level of values. If it cannot be decided by code, then it must clearly be decided by people.
Purpose of Locks
A blockchain constitution is like a lock. It is meant to keep honest people honest. Almost everyone knows the value of locking your door and they do this despite the fact that almost anyone can kick it in or find a way around it. When an honest man comes across a locked door, he will not try to open it without a key. This same individual may explore what is behind an unlocked door.
If the Ethereum Foundation, and key members such as Vitalik, had signed a carefully considered constitution then there would have been much less room to debate their actions. Many people have made the reasonable analysis that people's opinions would have been different before the DAO bug.
A constitution is like a good fence. It lets everyone know what lines cannot be crossed and gives intellectually honest people the ability to recognize when they have strayed from their prior agreement. If you join a community where the vast majority have explicitly agreed to a set of values, then it is more likely that community will stay true to those values.
Never Contract with a Dishonest Person
All contracts are worthless if entered into with a dishonest individual. You are almost certain to end up in court and you will be lucky to get your money back even if you win. A contract is there to remind honest people what they agreed to. Most honest people who are confronted with a contract they have signed will yield to the contract. Most of the time disputes are easily resolved. When there isn't a contract then disputes get much more complex. A game of "he said, she said" starts and the final outcome is unpredictable.
If you wouldn't enter a contract with a dishonest person, then you shouldn't enter a smart contract with a dishonest community. A community that wishes to maximize its value must earn trust. Trust can only be earned by consistently adhering to a set of principles. By having community members voluntarily sign a constitution it gives everyone involved increased faith in what will happen in the future.
Not Everyone Needs to Sign
A blockchain doesn't need a single constitution signed by everyone, but can be comprised by a set of constitutions signed by different subsets of the community. The goal is to document and communicate the philosophical DNA of the voters in such a way that others can easily decide if they would like to join. In theory, a constitution could be divided into many individual parts that are individually ratified.
To the extent that there is overlap among the variety of constitutions others can more heavily rely on those principles being adhered to.
When contemplating a constitution it is important to avoid issues that can be resolved by code and instead focus on things that keep honest people honest. There is no enforcement other than public shame, loss of reputation, and loss of market value. A community that violates its constitution will lose market value. The only enforcement available is the option to sell. The reason we adopt an enforceable constitution is because they are useful for convincing people to buy in the first place, assuming you can be trusted.