Yes, mercenaries and private military contractors are a good idea, because people will fight harder if they actually want to be in the army. When a person is conscripted, they may or may not really want to be there. A person who does not want to be there will not try very hard. Mercenaries and private military contractors are there voluntarily and will work harder.
Mercenaries are fine. I find it better to employ those who are fighting of their own free will, than those who are indenture servants. But the main problem with contractors is that they are employed, in our country's case, but the military industrial complex. So, yes they are good, because they replace indentured servitude, but bad because they don't solve the real problem.