I don't see a problem with PMCs. There are many similarities between them and regular armies, you get good contractors and bad ones just the same as with armies. The vast majority of the time these companies will do thier best to abide by the rules for the simple reason that if they do anything outright illegal they will be in trouble they can be fined, people can be jailed and it's not a good way to do buisness.
A lot of people seem to view PMCs as glorified hitmen, this is a bit unfair as a lot of them engage in protective operations ie. bodygaurding VIPs in Iraq and Afghanistan and of course as Darth Mobius said even the Pope. Most western PMCs only emply ex-special forces like SAS, SBS, Delta Force and Navy SEALs so they are extremely highly trained and as has been said on this thread before they are doing what they are good at. Best example I can think of is Sierra Leone. The countries official army was inept at dealing with the rebels so the government contracted a South African company called Executive Outcomes to supply a force of Mercs to help and within weeks they had turned the situation around and the government were back in, almost, total control.
Overall I would agree that they are a good idea based on what I have seen of Western PMCs. However, I am aware that there are many Eastern, ie Russian, Chechnyan companies that do not abide by international law and have been known to use questionable methods to achieve thier goals.