I feel like your austere meta-ethicist is mostly missing the point. It's utterly routine for different people to have conflicting beliefs about whether a given act is moral*. And often they can have a useful discussion, at the end of which one or both participants change their beliefs. These conversations can happen without the participants changing their definitions of words like 'moral', and often without them having a clear definition at all.

[This is my first LW comment -- if I do something wrong, please bear with me]

This suggests that precise definitions or agreement about definitions isn't all that critical. But it's sometimes useful to be able to reason from stipulated and mutually agreed definitions, in which case meta-ethical speculation and reasoning is doing useful work if it offers a menu of crisp, useful, definitions that can be used in discussion of specific moral claims. Relatedly, it's also doing useful work by offering a set of definitions that help people conceptualize and articulate their personal feelings about morality, even absent a concrete first-order question.

And part of what goes into picking definitions is to understand their consequences. A philosopher is doing useful work for me if he shows me that a tempting-sounding definition of 'morality' doesn't pick out the set of things I want it to pick out, or that some other definition turns out not to refer to any clear set at all.

Many mathematical entities have multiple logically equivalent definitions, that are of different utility in different contexts. (E.g., sometimes I want to think about a circle as a locus of points, and sometimes as the solution set to an equation.) In the real world, something similar happens.

When I discuss, say, abortion, with somebody, probably there are multiple working definitions of 'moral' that could be mutually agreed upon for the purpose of the conversation, and the underlying dispute would still be nontrivial and intelligible. But some definitions might be more directly applicable to the discussion -- and philosophical reasoning might be helpful in figuring out what the consequences of various definitions are. For instance, a non-cognitive strikes me intuitively as less likely to be useful -- but I'd be open to an argument showing how it could be useful in a debate.

Probably a great deal of academic writing on meta-ethics is low value. But that's true of most writing on most topics and doesn't show that the topic is pointless. (With academics being major offenders, but not the only offenders.)

*I'm thinking of the individual personal changes in belief that went along with increased opposition to official racism in America over the course of the 20th century. Or opposition to slavery in the 19th.