Omni-directional Spüeaker System Model GB.1Eīold in concept and unparalleled in its ability to reproduce soft, natural sound, JVC's GB-IE omni-directional stereo speaker system makes you forget you're listening to a speaker system. If you want to hear them, though obviously you won't get a omnidirectional sense because it's a video. The JVC's actually have 8 drivers, two 5 inch woofers and two 2 inch tweeters on the top and bottom half. Like so many things in audio, it's more how well executed the speaker is than what it's radiation pattern happens to be. Properly placed, in a good room, they can portray a very convincing soundstage.
As mentioned, the Ohm Walsh series, Shahinian Obelisk, and Wolcott Omnisphere come to mind. While there have been more than a few speakers that used the idea more for marketing than practical results, there have also been some very worth while speakers that were omni. To the OP's question, I don't think it's necessarily just a"gimmick". IMO, for a speaker to really be omnidirectional, it should be broadcasting most of it's frequency range in a roughly 360 deg pattern. At best, I'd call such an arrangement a "high frequency dipole". slightly more HF sound, over all, but allowing for a little less direct HF volume to keep the speakers from sounding too bright. Rear facing tweeters alone are generally meant to simply add a little "air" to the highs. I don't consider a rear firing "super tweeter" to make a speaker omnidirectional.