Murphy points out that for pretty much any (remotely plausible) moral theory, we'd be a whole lot worse off if no-one was complying with its requirements. It starts to look odd to condemn a theory as 'too demanding', then, given that the compliance effect on us is a net positive. One might turn to a comparative conception of demands, according to which utilitarianism demands too much of the wealthy because they would be so much better off under libertarian norms. But this objection may be turned on its head: "commonsense morality is very demanding on [a poor man] in virtue of the fact that he would do so much better under utilitarianism." (p.55)
P.S. Here's a delightful quote from p.60:
Our individual and social lives are so thoroughly structured by moral and political concerns that we apparently lack any independent perspective from which to examine the impact of those concerns on what they structure.
Though I'm even more struck by the point that our lives are "so thoroughly structured" by moral norms that most of the time we don't even realize it. (Consider, for example, the implicit interference that property gives rise to.)