As a sort of singer and aspiring musician , I know exactly where Dave is coming from ! IF someone has started a song , it is not really good form to join in and play it the way you want to do it , any more than joining in a chorus and changing the tempo ! The first thing a musician should learn to do is LISTEN ! IF you can play in the key the singeris in , and it is NOT a traditional song NOT designed for a small Orchestra , DONT play - If the singer is accompanying himself , join in QUIETLY .
And there IS a tendency for Instrumental sets to be played MUCH too fast to be danced to ! Its fine to 'entertain' an audience by doing a set of reels , each in a different key , and somewhat up tempo , but you CAN lose the feel of a melody if its played too fast !
So if we all step back a little , and listen to each other and try NOT to be either and ego tripper or prima donna , we can have good mixed sessions !