If you've got a really touchy job you need doing and don't want to be traced back to it by sending in your special forces, mercenaries are a good choice, since their only link to you is the jiffy-bag of unmarked notes you bunged them a few weeks beforehand, which you can brush under the carpet.
Alternatively, militias (the OFP resistance, perhaps) often buy in foreign experts to train their forces and help them achieve riskier objectives.
If anyone makes some mercs, it'd be an idea to give them a mismatched 'uniform' (for example, US BDU trousers, British DPM jacket, with a Russian load-bearing vest) and a variety of old favourites gun wise, such as AKs, G3A3s, wooden-stock FN FALs, Colt Commandos, Galils etc. etc. but without making them look really scruffy and non-military, as it's only fair to assume that you'd need to be a well-trained soldier with some form of military background to have the neccesary skill and balls to be a mercenary.