Speculative applications, i.e. application without a job announcement being published, 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 thus considering the pros and cons of this type of applications.

The pros are almost evident (more exposure). However, I worry about potential downsides. In particular, how would potential employers look at such an application? Could it make me look bad? Is there any other downside I haven't thought of?