Having duplicates should make the search better, by pointing people who phrase the same problem in different ways to the same place. But low-quality questions often don't produce something searchable for others, and they cover topics relevant to people who lack search skills.
Regarding duplicates and deletion you may be interested in my thoughts: https://meta.stackoverflow.com/questions/426214/when-is-it-a... ; https://meta.stackoverflow.com/questions/434215/where-do-the... ; https://meta.stackoverflow.com/questions/421677/closing-a-qu... seem relevant here, browsing through a search of my saved posts.
Having duplicates should make the search better, by pointing people who phrase the same problem in different ways to the same place. But low-quality questions often don't produce something searchable for others, and they cover topics relevant to people who lack search skills.