Lets use a simple example....
Say you have a $150k mortgage on your house at 4% interest.
You have saved $50k for a new truck.
The dealer has 0% financing on a new truck
The smart plan would be to buy the truck on credit and use the $50k to pay towards your mortgage, the numbers look even better if instead of saving the $50k you payed towards the mortgage as you went.
You would save 4% , now compound that and you have some real money.
Or you could pay cash for your new truck...... hmmmm
So it's not so black and white.... a million ways to skin this cat and everyone's needs and situation is different
