I simply don't agree with you sockamnesty
I'm not, nor do I consider that I should be, either interchangeable nor an equal parent with my exH. that would be ridiculous.
He is a parent for twenty four hours per week. I am parenting for the remainder of the time. I don't expect him to be an equal parent I just expect him to love, care for and engage with my children during the time that he has them.
I don't expect him to live up to my standards, he's not capable of them, that is why he is an exH and not a present one 
he has many redeeming features. he's arguably more irreverent, carefree and fun than I am. I dare say my children benefit from this difference between our natures.
picking out clothes, washing and ironing aren't his forte.
so I pack a bag for the children which he returns (mostly) on time.
No big deal. my children know he's a part time parent by virtue of the fact that they see him so relatively seldomly.
His flat isn't in any way their home, why would it be? its their Dad's flat where they stay once a week.
They live with me, their step dad and baby brother and sister. They have one home. Not two.
I don't understand why anything else should be the case.