A jacket doesn't really serve the same function as a cape because a cape/poncho protects the entire body whereas a jacket only protects the upper body. It's the complimentary pants which make the jacket/pant solution such a hassle, whereas a cape is as easy to take on/off as just a jacket. Plus a cape allows for more ventilation than a jacket ever could.

Granted, capes do slow you down and take some practice to handle in high winds (which is why I mostly prefer to just get wet and wear clothes that dry quickly/don't sponge up water), but, if you want to wear a particular outfit and need an outer barrier to keep your clothes dry, capes can't be beat IMO.

Mind you nothing is going to keep you dry riding long distances in heavy rain, so anything intended to block the rain is useless if it isn't easy to deploy for short jaunts of <10 miles.