I suspect that the average person - actually, make it 99.5% of the people - is better off with advice along the lines of "if you can possibly help it, don't get into debt to buy anything that can't be used to make enough money to offset the interest, and be very careful even " then "get into debt as long as the utility offsets the interest."

I think so because I doubt that most people (regardless of how good they are at math) can trust their current selves to compare utility from consumption to disutility from having to pay back (meaning that their future self will often disagree strongly with their current self.) This is not to say that someone else should be trusted with decisions about savings and loans other than the saver/borrower (forcing someone to put money into a pension fund on the grounds that they're irrational and then not really paying the pension back is a bit too common for my taste), just that "by default, don't get into debt" is good advice.

More generally, IMO quite often clever utilitarian maximization works worse than simple rules designed to keep you out of trouble first, and seek "utility" later. (I guess I don't quite believe in people being utility-seeking machines, not to mention ones having stable preferences, nor do I believe that a person is truly better off having achieved more utility per unit of time.)

I realize it's not a satisfactory argument, of course, if one does believe in utility maximization as a good way to think about these things, and I really wish I could back this up so it's more than a hunch...