There have been lots of bipolar (open back) designs over the years. Magnaplanars, Acoustats, Martin Logan, Carver, Apogee are planars. Infinity had a beautiful looking system using cone drivers.
Problem is that as the wavelength gets longer the evil wave from the back starts coming in phase with the good wave from the front. This results in a 6db/octave rolloff below the wavelength corresponding to the baffle size. In short, wimpy bass. That is why the hybrids exist (conventional cabinet for the bass section).
This isn't a problem in headphones. The whole situation gets turned inside out (visualize that one a couple of times) and now you are isolated from the evil wave by the ear pad, unless you have K1000s. In that case the proximity of the good side allows for reasonable bass response, but they still roll off a bit early. I will admit that I have tried a foam surround for my K1000s, and bass was enhanced dramatically. Getting them closer to the ear also helps. That experiment is incomplete.
As for the higher frequencies coming out of the backs of the speakers, it gives a sense of spaciousness and air. Some like it. I don't. I keep trying to figure out a way to get rid of the back blast off those Martin Logans, because I love them otherwise.
There are also Di-polar speakers out there. They add extra drivers on the back of the cabinet to get a similar effect. Bose 901s are the extreme example.