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.

That is an important point. People often run on examples as much as or more than they do on definitions,and if their intuitions about examples are strong, that can be used to fix their definitions (ie give them revised definitions that serve their intuitions better).

The rest of the post contained good material that needed saying.