If you can reverse a transaction for a good reason, you can reverse a transaction for a bad reason too. And since there is no central authority, your options are "anyone can take back their coins at any time for any reason" or "no one can take back their coins ever". One of these options is useful in the real world, the other is not.
Transaction risk, which is real, will be handled in the bitcoin world just like it is in the real world. By third parties that are paid to manage that risk.
True you can reverse transactions for bad reasons too, some charge backs are frauds, some are just careless judgements by the arbitrators. But fraud is a business cost that is a part of any serious business calculation. The question here is the acceptance and growth of bitcoin, without consumer's adoption, there is no bitcoin (practically speaking, not niche underground population).
The introduction of a third party to manage transaction risks will involve fees, which party will shoulder the cost? Right now, mtgox is eating the 25cents cost for accepting dwolla transfer, while tradehill makes the user eats the cost (which is really miserly and reflect their underlying mentality, which is why only a little faith should be placed on tradehill for responding to a crisis of their own).
99.99% of people in the USA perhaps. Please remember that USA != World and that Americans may be a large minority, but a minority nonetheless among Bitcoin users.
Today, many countries in the world face the threat of a currency crisis, including America. This isn't some crackhead dystopian fantasy. This is a real threat. How do I know this? Because even big corporations' CFOs are taking steps to be prepared for a possible collapse of the EUR for instance. Money doesn't lie.
I have experienced a currency crisis first hand when I was a child, where people lost more than half of their cash savings overnight. It is a traumatic experience that scars people for life. Those kind of people are emotionally attracted to a currency that can't be manipulated. Currency crises happened as recently as 2001 in Argentina and 1998 in Russia. Those aren't exactly small countries.
Americans perhaps don't feel that way because they live in a bubble. They feel that just because something has never happened it will never happen. If (when?) a currency crisis hits the US, it will take everyone by surprise, and it will be ugly, and then ordinary people will come flocking to alternative currencies.
The education of the average american or citizens of other developed countries is shockingly disappointing compares to the technical advancement of modern man. With that said, the technical sophistication of an average American is still vastly superior to the citizens of the developing countries. If an average American can't use bitcoin easily, what makes you think an average russian/argentinian can do better? If an average American who makes $50K/yr worries/bitches about losing a $20/1btc due to fraud that can't be reverse, I am sure an average argentinian who makes $20K/yr would worry more than an average American if the same 1btc is at risk.
You say that bitcoin can't be manipulated, but how can you say that knowing (or reasonably suspect) that 2 or 3 early adopters have control of at least 10% of the total float?
And so far, bitcoin has been proven to be risky and unpractical in its current framework.