Speculative applications are common (well, at least normal) in non-academic jobs. But I am not sure the situation is the same in academia. Somebody has suggested me to send speculative applications to different departments for teaching or research jobs. I was wondering what are the pros and cons of this type of applications? The pros are almost evident. In fact, I worry how the potential employers look at my application and how they are going to consider me in the future. Is there any downside for job-seekers in academia to send speculative applications? 

Before ending this question, I appreciate if you share your personal experience or your insights (suggestion) about how to prepare a speculative application.