lol @ grammar
the US is using "mercenaries" right now in Iraq. The "civilian contractors" are the security firms that are in essence mercenary companies. Obvously these guys don't just fight for anyone but by the strictest definition they are mercs.
And I agree, it really depends on the situation.
Sometimes when it's low risk, yet important and you want to get it done quickly and efficiently (like training foreign troops) you might want to use these guys because they are professionals, they're effective and they don't brign along the military red tape.