No, we don't. If in the debt contract it is written that only bitcoins are accepted as payment, that clause is legally void: the debtor may still pay with legal tender if so he wishes.
Im not a lawyer, that may or may not be true, its the first time I hear it. You are essentially saying barter contracts are null and void? that would surprise me. What I can see is if only one part of the barter is or can be executed, that the resulting debt could be settled with legal tender; that is, well, the whole point of legal tender. If we agree to trade a laptop for gold and your laptop gets stolen after you got my gold, you will owe me the gold, or absent that, legal tender to compensate the breach of contract.
But that wont prevent two parties from bartering if they so wish, and its not something Ive heard Keynesian rage against.
That's how legal tender works. If someone delivers on their half of a contract, the other party can legally get away with paying in or with exchanging legal tender instead, even if the contract specified an exchange in gold, bitcoin, chocolate chips, or even another nation's currency.
The problem this leads to is that it effectively eliminates those types of non-legal-tender-accepting contracts from the market. People can sign on the dotted line
saying they'll pay in gold, or that they'll barter toothpaste for the product, but they can be fully intending from the start to ignore that and pay you in paper. And legally there would be nothing the seller could do about it. That's the problem with specifying a method of payment in the contract. It's unenforceable, and typically meaningless. Even though the occasional participant will honor it, there's no (legal) obligation to do so, and over time fewer will.
And Krugman, and economists with similar beliefs (Keynesians,) believe in legal tender. In fact, they assume it, and much of their theories revolve around the presumption that the government will force a specific monetary system onto the people, especially through forcing people to accept that government money even if it's not in their contracts. They don't rage against barter because (1) their theories already presume an environment in which barter will realistically be unenforceable, and (2) they don't actually want to come out and plainly say that they dislike barter, and want it to be unenforceable because they know how something so intrusive will sound.