The assumption that morality boils down to utility is a rather huge assumption :-)

It's not an assumption; it's a normative statement I choose to endorse. If you have some other system, feel free to endorse that... but then we'll be discussing morality, and not meta-morality or whatever system originally produced your objection to Jiro's distinction between good and bad.

on having a good definition of "morally"

Agree.

An omniscient being has no risk and no risk aversion, for example.

Well, it could have risk aversion. It's just that risk aversion never comes into play during its decision-making process due to its omniscience. Strip away that omniscience, and risk aversion very well might rear its head.

Morality is supposed to be useful for practical purposes. Heated discussions over how many angels can dance on the head of a pin got a pretty bad rap over the last few centuries... :-)

I disagree. Take the following two statements:

1. Morality, properly formalized, would be useful for practical purposes.
2. Morality is not currently properly formalized.

There is no contradiction in these two statements.