Perhaps, but if that user is hacked due to their weak password, while it would technically be the user's fault, it would be the site owner who would take the blame for it in the user's eyes. It's better to protect the user from themselves.
And "just" a commenting platform doesn't quite cut it for me, people can take their online reputations very seriously. How would it affect the HN community on a whole if tpatec or michaelochurch (insert your own respected user here) were hacked and their profiles destroyed?
You are looking at this from your point of view. Someone who values his online identity and (probably) already takes the required steps to protect it.
At the other extreme, my father commenting on the news couldn't care less if his account is hacked. All he wants is a fast and easy way (using the same password he always uses) to post on the Internet, with a relatively secure way to identify himself to other users on the site.
You need to sit next to someone who doesn't live on the Internet to realize how simple measure, such are requiring a minimum password, can really piss them off.
"Oh, my account got hacked... I guess I'll make another."
"God FUCKING DAMNIT this stupid website won't accept my password!"
And "just" a commenting platform doesn't quite cut it for me, people can take their online reputations very seriously. How would it affect the HN community on a whole if tpatec or michaelochurch (insert your own respected user here) were hacked and their profiles destroyed?