Mercenaries in the medieval period are an interesting thing, they are well studied in the Italian pennisualr and the south of europe but somewhat neglected in other areas. One of the most significant turning points in English medieval history (Post 1066) was the reign of King Stephen, and the wars of succession. Mercenaries were increidbly important in his early campaign and it was division between the mercenary and feudal forces of stephen's army that fundementally weakened his hold on Normandy.
I personally believe it's very important to distinguish between travelling bands of Knights who may change loyalty and allegiance, and mercenary bands. Although both were often reliant on payment being a mercenary is almost always recorded in written works ina negative light. Bands of mercenaries are often reported for their detrimental effect on relations between whomever they are serving and where ever they are placed. The behaviour of mercenaries may be recorded as looting and plunderign while that of a band of knights would be viewed as a chivalric act. In instances the two assimilate and change depending on culture and language, but they have intrinsically differnt identities in the medieval period.
I try to avoid referring to anyone as a 'professional soldier' in the medieval period, as it is more an expected way of life than simply a profession. The orders of medival society in their own imaginign are those who pray, those who fight and those who toil. Those that dared to enter into the realm of war were often the subject of ridicule at the hands of the aristocratic powers.
I think drawing any comparison from a medieval mercenary band to a modern PMC, although amusing, is fairly pointless.
Check out William Urban's 'Medieval mercenaries: the business of war' (2006) for some great insight into their importance, and how perceptions of 'professional' soldiery changed in the medieval period.
