There is nothing wrong with "spontaneous fun". However some people like to play music well, and hear it played well. It is perfectly possible to do this in a session, but problems arise when the two approaches try to combine in the same session.

Good percussionists can add to the music. Unfortunately they are vastly outnumbered by those who just see it as a way of joining in. Far too often good percussionists are forced to stop playing when the thumpers and bashers join in, because they can no longer make a contribution to the music.