Bags I don't want to argue but your wrong I have been painting many years and I can guarantee you don't have to do a half level 5 to look good. First if what you are saying was true every job would have to be a level 5. Second level 5 is a lot more than a skim coat that's why a level 5 costs at the least double the price of a level 4. Primer is your skim coat on new drywall it's job is to make the difference in mud and the paper the same. The main reason for a skim coat is to repair damaged drywall.