rear firing tweeters....

Some of the Revels use them; I am sure there are others. As I sit IN FRONT of my speakers I have never noticed a need for them! Just kidding.

I have a pair of VS2 Von Scheickerts which do have an "ambience tweeter/?high midrange " driver. It does see to add space to the sound but not much more so than a good tube output power amp qould ( I use both). It doesn't seem to hurt that's for sure. My main speakers, in a different rig, have better 3D halography (Focals) and don't have them. From that little experience I conclude they are not needed in all cases, the VS may be making up for a particular issue of their own.

I do like that the Von Schweikert rear firing tweeters on the VR-4 and other models have attenuators so that you can adjust the level to better integrate the speaker in your listening space (or your listening preferences).

My satellites have rear-firing tweeters as well as the front ones. It certainly adds to the spacial qualities of the speaker system, but the stated goal of the designer is to even out the in-room high frequency response.

I had those Snells. E-IV, I believe? I liked them, but not necessarily because of the ambience tweeters. I think they were just good, high-value speakers.

To me, it seems a rear-firing tweeter is basically a reverb unit. There will be some delayed arrival at the ears, delayed enough to not cause phasing errors, but perceived more as an "echo". Ultimately, I'm not really comfortable with intentionally smearing the HF response in time to add "ambiance" beyond what is captured on the recording (or not).