Imagine that a reader gives you a dollar’s worth of trust by reading your book. That trust means she expects you to give her certain things like a good story, interesting characters, and competent craft, among other things.
Every time your story fails in one of these elements, the reader takes away a bit of that money, and, when there is no money left, the reader tosses the book without finishing it and will no longer trust you enough to buy the next book.
The really tricky problem is you don't know what will irk each reader. Maybe it's grammatical mistakes. She may take a nickel out of that dollar, or, if she really hates grammatical errors, that error may cost you a quarter or the whole dollar. Or it could be plot problems, bad science, or faltering viewpoint to lower that dollar to nothing.
Do you really want to risk losing that reader by being sloppy about any part of your writing? It's just pretend is not an excuse many readers will accept.