What seems to be the most difficult situation is for stepmoms who are put in the position to care for stepkids while the dad is at work or doing other things. Which is a position that happens all the time for a lot of families.
My husband's visitation schedule is centered around HIS availability, not mine. And I think that is as it should be. His time with his daughter is HIS time with his daughter. It is time for them to bond and enjoy one another. I was clear with DH about this from the very beginning. While I do have a relationship with SD, I am not her caregiver and never have been. When he is unable to be there 150% for his daughter, then she is with her mother (or whatever childcare provider her mother chooses), not me.
The ex once asked me to watch SD (on my day off) because she was unable to find child care (DH was deployed in Iraq at the time). I agreed to do it, but it never came to fruition. I had asked her to send SD with some snack food, since SD hadn't been to the house in 6 months (because of the deployment), and there wasn't much in the line of kid food for her. I thought it would be nice for SD to have some familiar food items with her. Apparently, this was too lofty a request and the ex called it off at the last minute due to what she viewed as insolence from me.
If I'd had any reservations about my decision to hold my ground as a separate entity (not an additional caregiver) before that point, the experience with the ex served to solidify my early concerns. I will not put myself in the position of having all of the responsibility of being a parent, but none of the liberties or the respect (Or in some circumstances, the boundaries, safeties, or social understanding). I expect her parents to parent her.