This answer refers to the practice of over-riding the fast of the first-born, which is my experience too. My question is: should the shaliach tzibbur ever do the fast-day liturgy? If the community has a planned siyyum+meal, do we assume that everyone will attend (or, having had the option to, is on his own if he wants to fast)?
My morning minyan doesn't do fast-day liturgy on that day but I haven't found anybody who knows a reason beyond "that's what we've always done".