So here's a question for anyone who thinks the concept of a utility monster is coherent and/or plausible:

The utility monster allegedly derives more utility from whatever than whoever else, or doesn't experience any diminishing returns, etc. etc.

Those are all facts about the utility monster's utility function.

But why should that affect the value of the utility monster's term in my utility function?

In other words: granting that the utility monster experiences arbitrarily large amounts of utility (and granting the even more problematic thesis that experienced utility is intersubjectively comparable)... why should I care?