the mac OS is paid for and the reason i tried out Parallels with Ubantu, 64 bit , was because it was free and I did not feel like spending time with parrots, cutlases and unusually named bearded men.

The Ubantu installation is fully installed so it can save and load files & programs, it is launched from the Parallels application. The Parallels app creates a 35-ish Gb virtual drive (don't ask me how that works, it has not changed any of the available drive space numbers ) and instals there. The people over at Parallels are naturally not making any mention of Linux, instead opting for a marketing strategy that promotes their product as an alternative to the mac dual boot system.
Ubantu has been a bit slow to react sometimes, maybe because it has to share resources, (on a macbook with 4GB RAM and 2.1 Ghz processor) but switching from one OS to the other is as easy as ctrl+tab - Ubantu is in reality running as an application within the mac OS.

I will add MS to the mix at some point but i have to get a copy first and I want a legal one, I only pirate when I can justify it