In, This Is Why It Matters if Your User Guide Is Just an Afterthought, Bill Kerschbaum posits a scenario in which a potential customer, impressed by your glossy website, downloads a trial version of your software, is initially impressed, but then tries to figure out how to do something, is disappointed by the poor user manual and decides not to buy the full version.
My immediate thought on this was, but that is not what happens today. People don’t turn first to the user manual. The first thing they do is Google or ask their social network how to do something. Unless your user manual is online (preferably in the form of Every Page is Page One topics), and unless it ranks reasonably well in the search results for questions about your software, it isn’t even going to get a chance to disappoint. Instead, whether the user decides to buy your software or not is likely to depend on whether some other user has documented how they did the particular task they are interested in.