Restarting all the time to boot into different OSes is pretty tiresome and unoptimal (time-wise). Running XP inside Vista, or XP inside Ubuntu sure has a certain comfort to it, that dualbooting does not have.
If the topic interests you, you might want a read at
this, as a start.
Personally, VMs can be pretty handy when you freeze them (allows you to inspect the whole memory range or that of specific softs, cool for finding out 'hidden stuff' that actively hides its presence)