Here in southern France, while there are not enough sessions as we are out of the central core trad area, the very best ones happen like this. As the music played is as you say, 'played right' - ie it's dance music so play it so it can be danced to - there will be an area where people can dance if they want to.
This is made a bit easier in that most French dances are couple dances - scottish's, mazurkas, bourées, with the occasional circle or an dro.
The demand for the music to be danceable can help avoid the flat out thrash that sometimes happens. It also however allows for something different to be played after a particularly energetic dance.