The authors wrote these stories the way they wrote them for a reason, and eventually the children will catch on. However it is a shame that so many people are overworked and overtired, so perhaps things need to change so that parents are not always so tired and people can spend time with them.
Yes, stories are necessary for children, and parents are therefore doing them a disservice. How will children learn to tell and appreciate a complete story if they are not instilled with these values early on? A parent should instead put the book down when tired. They can return to it the next day.
Although the children may not be getting every single word of the story, they can still understand the plot as long as the parents are reading the beginning, the end and the most important parts of the book. A parent is able to cut corners for their own sanity without having to feel like they have caused a disservice to their kids. They can read it themselves if they want.
Parents that skip pages for bedtime stories are not doing their children a disservice. A study found that one-third of parents admitted to skipping pages because they are too tired. Children still get the benefit from reading and spending time with their parent, even if it is short changed a bit.