[giantg2]

"But the average person has much more marginal utility for their amount of wealth."

Not while it's invested and locked away in a 401k, according to your comment about it not being used (presumably consumption based):

"Wealthy people aren't using money that society could put to great use." Emphasis mine

"What else am I missing about utility in this context?"

This thread was in response to someone claiming money has less use when you have more of it, which isn't really what marginal utility is about.

"At a much lower rate than income, further cementing the advantages of the ultra-wealthy."

Ok... we aren't really discussing rewriting the tax code.