Personally, I think mercenaries perform a valuable service to society. Take the example of Sierra Leone for instance. Back in 1998, the legitimate government of Sierra Leone paid British Mercenary group Sandline International 20 million dollars to take care of the rebel problem. After Sandline International had properly disposed of the rebels, thereby solving most of Sierra Leone's problems, the United Nations stepped in with a peacekeeping mission.
This peacekeeping mission ended up costing the UN 1 BILLION dollars, which was 5 times the price tag of Sandline International. If you ask me, that was a total waste of money, as the UN did nothing for Sierra Leone except come in and steal the glory after Sandline International had solved the problem. Anyhow, the bottom line is that mercenaries can do the peacekeeping work of the UN for only a fraction of the cost.
In addition, mercenaries can also step in when political leaders need to achieve certain goals and objectives, but are unwilling to commit state military forces due to limited political will. Such is the case in Colombia, where many civilian companies such as DynCorp and the Aviation Development Corporation are being contracted by State and DoD to take care of GW's anti-narcotics agenda.