John Robb says “the US military is completely dependent on private military companies” like Blackwater and this trend will increase and can’t be reversed. Why? Because PMCs are “efficient, scalable, and contingent.” In other words, they get hired for the same reason any contractor gets hired. Especially by bellicose governments fighting too many wars with too few troops.
The only way to stop them and their growth is by ending the wars they are hired to fight in.
Machiavelli was noted in the comments. He said mercenaries are never good because they are either militarily ineffective, and thus useless, or they are militarily effective, and thus a threat to you.