Are Ripple IOUs fungible?
That's just reality. IOUs will be valued differently depending on who you got them from.

You probably value $100 in cash higher than the fact that your friend Joe owes you $100. You can use the cash right away while you have to wait for Joe to pay you back or you could transfer that debt to some common friend of you and Joe's (a friend who is fine with Joe owing him $100).

Now compare it to having $100 in a bank account at your local bank (i.e your local bank owes you $100). You'll probably value that a bit higher than having Joe owe you $100 since it is easier to instantly transfer it to your friends that have accounts at the same bank.