That is one of my driving pet peeves too!
Like, gee, now that you have slammed on the brakes and nearly caused me to swerve into a ditch, it would really be helpful for you to flick on that signal NOW...hmmm I think the point is signal first, hence warning other drivers what you are about to do BEFOREHAND. Or else the ones that slow down to a crawl while you are trying to figure out if their car is dying and whether you should pass them or not and then just when you are sick of trying to anticipate what the heck they are trying to do, they finally see your exaggerated handgestures in their rearview mirror and flick on a signal. Freakin' geniuses.
Whew. I feel better now. Thanks Tiff. LOL.