I was surprised to learn that iOS 4 - 8 permits updating the firmware without first entering the PIN.

Device Firmware Update mode https://www.theiphonewiki.com/wiki/DFU_Mode

This technical factoid is relevant to the current discussion. I did not understand how creating a custom firmware was useful.

To further ground the discussion, I found this informative:

Legal Process Guidelines U.S. Law Enforcement

https://ssl.apple.com/privacy/docs/legal-process-guidelines-...

Whereas before I was firmly against Apple helping to crack the San Bernadino iPhone, I'm now merely mostly against.

I don't understand how this action can help FBI. What additional, unique information could possibly be on this phone that they couldn't discover by alternate means?

I don't understand how an error by FBI obligates Apple to clean up their mess.

My understanding is that iOS 9 changed things so that this kind of forensics backdoor is no longer possible, mooting this discussion.

FBI should take their lumps and learn from their mistakes.