Time coherence - how important and what speakers?

Sure time and phase coherence are important, but these are only two aspects of many that go into how a speaker sounds. If a manufacturer achieves time/phase coherence but botches the crossover or driver integration, cabinet, etc. the result will not be pleasing. Gotta look(& listen) at the whole package, cause that's what you're ultimately hearing. I definitely hear something a little different when listening to Thiel speakers in terms of image specificity and a sense of dimension, but I don't know how much of that is due to their time/phase coherent design. Along with those mentioned above, Meadowlark is another company the adheres strongly to this design philosophy and much can be read about it on their website.

Audiokinesis - Unless I misunderstood your post, Thiel incorporates the tweeter inside the midrange in a concentric design in its 2.3 speakers (and maybe others) and I believe they also use a first order crossover -- so the design does exist. The woofer is a seperate entity though.