No they don't. We have already determined that you won't get IOUs from anyone you do not extend a trust line to.
And that's a feature, not a bug.
Therefore the IOUs are not generally tradeable.
Of course they are, that's the function of gateways, to issue and redeem IOUs that can be traded between any pair of individuals who trust the same gateway. This is very similar to what banks and payment processors do. You deposit cash with them and are then able transfer money between accounts without having to withdraw immediately.
If you guys don't understand the system then ...
Don't criticise something you clearly don't understand.