Like anything, it depends on the situation.  In the approximately 220 nights I've spent canoe camping in the BW and Quetico, I've shared a camp site with strangers exactly 4 times and brush crashed (camped where there was no campsite established) 1 time.  Only once was it not legal in the sense that more than 4 canoes/9 persons were in the same area at the same time.

In the karmic sense, I've offered to share a site 6 times, and only had 2 acceptances.  Wind, lack of available unoccupied nearby (within 2 miles) campsites, darkness, or threatening emergencies are all great reasons to share a site or camp in non-designated site.  If my criteria are met, I would gladly share a site in the BW because so often the sites are huge and we only have 1 or 2 tents, meaning another smallish group could easily fit.  Plus, it is closer to the BW's philosophy of concentrating disturbed areas to designated sites.

In the quet, it will take a little more for me to share a site since you can camp "anywhere".  I've pointed out campsites to travelers who were not as well prepared/familiar with the area, since maps rarely show all the established sites.  Finding your own site is part of the Quet's experience.  That being said, if you're freezing, wet, tired and can't go much further, not only will I roll out the welcome mat, I'll stoke up the fire and offer you some food.  The same has been done for me when I was in a tight spot.