The only effective circuit breaker is some form of synchronized method, but will the different exchanges cooperate? They really should put in circuit breakers this week across all exchanges so that when really bad stuff the panic doesn't destroy tons of people. It will be bad, but at least if the trades can complete and things unwind in a moderately slow fashion, it won't just 100,000 attempted transactions per second with random successes.
In the situation with ETH, a bunch of automated margin calls happened because of one large order on the books, dumping the price down to 10 cents! GDAX/Coinbase repaid a lot of people who lost money then, even though they didnt have to. They may now have some dumb heuristics that slow down trades / whatever if a bunch of large orders are going through, or price delta over a short time period goes too high. I remember looking at the charts today on GDAX and the 5 minute candle went from 20k open to 16k close before trading halted! I also heard from others that there were a lot of ~19k sells of 100 coins each on the books and executing.
So yeah, just speculating that this may bave been the case of someones boss at GDAX/Coinbase telling engineering "please don't make us do that again", after they ate the cost last time! :-)
"Decentralized! Deregulated! But coordinate to protect us from ourselves, don't let us lose money!"
That said, while I don't think the exchanges need to (or realistically could) coordinate on circuit breakers, I do think it's entirely reasonable for there to explore this type of mechanism on a per-exchange basis in the case of GDAX and other exchanges where margin is allowed, to prevent the whole "cascading margin call" thing that happened with ETH to happen again. That could take a lot of forms, including limiting the size of market orders to not eat 30% of the book, or not allowing 1 market order to trigger margin calls if (insert criteria here)
(That said, they could also just say "that's how margin and trading goes on low-volume, highly volatile assets" and technically be correct, but much like their decision to reimburse some traders, this is more of a business decision and wanting to keep your customers. Not sure what the "right" answer is.)