I can't see how it is worth discussing this in general terms because there are as many different situations as there are people inolved.
In our case, dp pays ex an agreed sum each month for child maintenance, above the csa amount. Yup, fine. She got all the equity and all debts paid when they split. No problem.
Dp then pays for all school trips, holidays, pocket money, phone top ups, all clubs (dss does four different things), kit for all clubs, takes him to all activities, arranges and pays for all birthdays, buys school uniform and clothes to keep him kitted out at our house.
The ex almost never sees him. He lives with us. She works full time. In theory, she should be paying us. Over the whole Easter holiday she had him one night, two nights over the whole of Christmas break, two nights last summer holidays. There is no routine. She calls off continually, letting him down. She says it's because her new partner is "difficult". Oh, and she has asked for her maintenance to be increased "in case she splits up with him".
Dp is just doing the right thing by dss. He doesn't want to be in conflict with the ex. Quite right too (though I draw the line at her coming to our house and telling us we need new sofas!).
I think people whose partner pays their ex and who then go on to have more children they actually can't afford to support are idiots I'm afraid. There is only so much money to go around, all the children need to be supported.