Re: The ferry should have blown up
While I agree with the title of this post, I think the reasoning for not blowing up the ferries is two-fold. First of all, you have to take into account one of the main ideas in TDK, that Batman is essentially bad for Gotham. The psychopath who terrorises Gotham and is constantly one step ahead of the authorities is the direct result of his influence. Then you have the guys in hockey-pads who twist Bruce's reasoning for creating a symbol to inspire people (much like some of the posters here twist Bruce's TDKR line, "Batman could be anybody"). And then, of course, Dent's fall is practically his fault as well, considering the choice he made following the interrogation of the Joker. TDK is such a thorough deconstruction of the idea of Batman it's not even funny.
But behind all that negativism is the ferry scene and what it says about the people of Gotham. It is essentially a vindication of Bruce's objective, the one he details in BB (during that plane scene Nolan's been mentioning in his last interviews). It's the ferry scene that sells the fact that the people of Gotham are indeed worth saving (it's the main reason I can let the absence of the citizens' POV in TDKR slide) and that Batman's crusade is not in vain after all. It's also an indicator of Bruce's faith in people (the one Rachel hopes he won't lose), the one that comes back to haunt him with Talia, but also the one that saves him from being shot in the face by Bane.
Like BatLobster says, TDK is already dark as it is. The ferry scene is one of the few rays of hope within the movie and that's why I think it's essential. Another one, IMO at least, is the fact that Batman does prove himself to be the better man in the end (in comparison to Dent), because he doesn't break under the pressure of all the **** he's going through in the movie. But this last point is perhaps more debatable. As for the original point of the topic, no, I definitely do not believe the ferries wouldn't have blown each other up in real life. But I think that's the point Nolan and Co. tried to get across...if you don't take a chance and put your faith in people, good things will not happen and your perspective will remain bleak. TDK is paradoxical in this sense: it's really cynical when you think about it, but at the same time it has a very subtle, underlying note of optimism. (it's one of the reasons I think it's the best in the trilogy)
It's TDKR that shows us that this cynicism when it comes to people will only lead towards a single path: death. Ra's, Bane and Talia are essentially the antithesis of Bruce when it comes to having faith in people. Their extreme beliefs drive them to attack Gotham and attempt to destroy it, even though there are good people in the city (as TDK establishes). This is the staple of these characters, their rigid and staunch faith in their beliefs. This contrasts greatly with Bruce's underlying optimism and faith in people. It's why I think they work well as villains, because they tell us something about our protagonist as well. Batman villains are meant to be dark mirrors of Batman himself, after all.