The phrase "kill your darlings" circulates in fiction writing schools. The reasoning is that a "darling" turn of phrase which the author really likes is likely something that they are irrationally obsessed over and that distorts the editing process around itself, to the detriment of overall quality.
Like a lot of writing advice this is really subjective.
Again, subjective. Some people like it and it can be a valid literary art form in itself. It's only in purely utilitarian text like technical writing where it doesn't belong.