Once we realize a few things, I wonder how much else discussed here will be important. Those things include:
a) that we can describe our desires as properties of physical brain states and others' desires as properties of physical brain states,
b) that any of a set of activities would fulfill those desires,
c) that any of another set of activities would change those desires to those corresponding to other brain states, i.e. there are fact about how people will respond to stimuli. There are facts about which arguments people will and will not find persuasive at different times and under different pressures.
d) people actually mean things when they talk and this generates meaningless fake problems when their meaning does not correspond to their words (broccoli questions go here),
e) people think they think things that they do not think,
f) the moral systems people believe in and think they believe in form an incredibly complex conglomerate into which all beings' actions fit morally,
g) people may believe in square circles or a certain definition of "free will", "maximizing utility", or "morality" but that does not mean they can exist without internal contradiciton,
If someone asks me "Why should I be moral?" I can (in theory) tell them many true things. I can tell them if they are using the word "moral" to represent a coherent concept. I can tell them what they care about and why, as well as what it would take for them to care about different things. I can tell them which of those arguments that would convince them are invalid and/or untrue and which arguments they reject that are valid and true. I can tell them the relationship between their biology, what they care about, what they think they care about, and what it would take to change their desires.
In the midst of all this I don't feel poor for not being able to tell someone "why" they should do something. If I know how to make Biff my slavish handyman, and that certain people can't be convinced to be harmless due to their religious upbringing, and how to change the minds of others, and what mix of desires in the population results in honor killings and that all but the tribal and addled abhor that practice for good reasons, etc. please tell me what, if anything, I am missing out on by not being able to convert an "is" to an "ought".
I don't know what is left for "ought" to be about that is actually meaningful. And perhaps an error theorist like Russell Blackford would say "that's just the point!" And maybe so. But he also says this unanswerable, unfillable "ought" is the very thing laypeople address themselves to in their everyday moral discourse. I'm not convinced this is true.