Given all the reasons to send a cover letter, is it ever a good idea not to? “If the application instructions expressly say not to include a cover letter, or if an online application offers no opportunity, then you can forego the cover letter in these cases,” Atallah says.

This is most cases now. You can go into any library and buy a Killer Cover letter book off their discard pile for a buck now.

I think I have written two cover letters in the past ten years.  In my industry, most places don't want them.