By definition a hard fork erasing the DAO hack from the blockchain would mean that the blockchain was not immutable in this instance.

There are some persuasive arguments on both sides that Ethereum miners (voters) will have to balance:

The importance of an immutable blockchain versus the desire to refund innocent victims of the attack by using a hardfork to recover the stolen Ether.

Is "fairness" more important than an immutable blockchain? Will future currencies or DAPPs that care greatly about immutability think twice about building on Ethereum if the miners approve for a hard fork now? Will regulators use evidence of a hard fork today as support for pressuring developers/miners to hard fork again in the future?

If the price increase of Monero the day the Ethereum hard fork solution was presented is any indication, there is likely to be an exodus of some Ethereum users/developers to more immutable/fungible alternatives if a hard fork occurs. Many blockchain purists believe in immutability more than almost anything and will cite that MtGox hacking victims never asked for a hard fork to refund their Bitcoin losses.

The great majority of Ethereum users do not think it is fair that innocent DAO investors were robbed by a hacker and truly feel bad for the victims. The economic value of the hack truly was substantial (although perhaps less in % value of ETH market cap compared to the % loss of the MtGox hack compared to BTC market cap at the time).

Should the value of the loss dictate whether or not a hard fork is required? Should miners be worried about a slippery slope and requests for more hard forks in the future from victims of fraud, hacking, or regulators?

How will the miners vote? Right now there are more questions than answers.