What are the pros and cons of Private military contracts (PMC) or mercenaries?

There is simply no way at all that the State Department's Bureau of Diplomatic Security could ever have enough full-time personnel to staff the security function in Iraq. There is no alternative except through contracts...
The capability and courage of the individuals who provide security under contract is worthy of respect of all Americans...
...[O]ver 30 of our contract security Americans have been killed keeping the rest of us safe."