I think in our industry a cover letter might send a negative signal. In other fields a cover letter is a sign that you’re a “go-getter”. A good software engineer is two things: in high demand, and a professional problem solver. A cover letter sends the signal that you are the type of person who works harder not smarter. I’d be much more impressed if you LinkedIn stalked me and sent me a quick two sentence message about why you would be jazzed to work at my company (I.e problem space and why or culture fit and why).

On the whole it doesn’t really matter though.