One of those reasons is the precedent it sets of collusion. What happens if a government wants to intervene and threaten the miners to fork because they THINK there is a homeland security issue? What happens if they want to roll back just because they feel people aren't paying capital gains taxes?
That's what I think is going to happen. American regulators don't listen to ideological arguments (except for the ones that squarw with their own ideology) but they do listen to practical/technological arguments. One of the big reasons why they've been kept at bay from the crypto-only sphere, over and above the small-scale nature of this space, is the fact that a blockchain rollback is
very messy with Bitcoin's chains of tx outputs.
If the Ethereum folks can show the wold that a "surgical" excision is possible with a blockchain-altering hard fork, that infeasibility argument becomes dubious for the Ethereum system and its likesake.