This is the point I've been trying to get across to a lot of my peers. It's not always about you, but your audience. I personally fall into the "casual reader's category". You know how many books I've dropped or can't even begin to read because the language is not simplistic enough or the author spends 3 pages describing a chair? When I read, I want to be entertained by the plot, idea, character. I don't want to be reading a book equivalent to a TV documentary. Getting back on topic, I can imagine "the summer months crawled by like years" used in a scene where the scene is more important than the action/plot. The extra words put emphasis on the scene. It adds rhythm and sets the mood more, if that is what you are trying convey. I agree with the saying, “Less is more” in about probably 90% of the novel. But sometimes, when you want to emphasize something or set mood (although there may be other ways of doing it than using redundancy), sometimes “More is more.”
Extra words could be used to emphasize sentence, but why not just choose better words? Also writing like that can be a pain when you are dealing with strict word limits. Going back and pruning can help you stay under that 4000 word limit. I know people have their styles and all that, but I am speaking from a publishing standpoint, which is where this editor is coming from. I thing we all want to get are stuff out there at some point.
It might be because generally summer consists of sveral months and so to use the word "months" is regarded as redundant. I do agree with you though that this sounds odd. the word "like" implies that years crawl. They may seem to crawl but then the sentence starts to mean not only that summer seemed to last for years, but that it seemed to last for years and that the years themselves seemed to pass very slowly. But, the fact that she likens the passing of summer to that of years already suggests a slowness and a crawling that is conveyed by the word years and so the word crawl[I is not needed. Could write. "The summer seemed to last for years."