Bitcoin is, in a way, backed by the value that it brings as a currency-- privacy and anonymity. We can argue whether that value is $10 per BTC, but I also don't think any reasonable person thinks that it is zero.
EDIT: The fact that government would have a very difficult time shutting down the Bitcoin economy is yet another example of its utility and value.
What I, and others, are saying is that in the future, no such people will exist. Our prediction is that the current bitcoin economy is transient and will eventually vanish. Hence, we say bitcoins have no value. It's similar to saying that a particular stock has no value if you think that that particular company will collapse.
Not really. They have to be willing to say "I'll give you X thing that you value in exchange for Y bitcoins." The entire point is that the design of bitcoin makes that scenario likely to always be true precisely because it is nearly impossible to stop. As I said above, the very fact that it represents a system that is very difficult for state actors to attack ensures that it will almost certainly be used for exchanging goods and services that governments would rather not exist. Not a terribly noble future, perhaps, but certainly one that ensures bitcoins have value.
For a reason that I am unaware of, you seem to think that just because something can be traded it will be traded. That's not true. People tend to only trade things that are readily exchangeable in the larger economy - things that have value.