Peace of Mind does have high value ... depending upon careful thought as to what it is that provides the peace of mind.
I have a hard time wrapping my head around a hypothetical situation like this: Having $100,000 in an (insured) investment account earning 4% (capital gains over time) and also having a $100,000 3.5% (deductible at tax time) mortgage on the house and then cashing out the investment account to pay off the mortgage.
Peace of Mind to me means as much as possible not missing wise opportunities to optimize one's resources.