There are MANY tradeoffs associated with single driver speakers, so that whether the offsetting positive attributes make them desirable is largely a matter of taste and how a system is used. The most common deal stoppers for most listeners is the limited bass response, peaky upper midrange, and limited volume level. I have heard some very nice fullrange systems, but, on balance, I have not heard one that I would prefer over a high efficiency multi-driver system. I have not yet heard the Voxativ system which I have been told is one of the best at overcoming most of the problems with fullrange systems.
What I have been very impressed with are systems that use one driver to cover a substantial part of the frequency spectrum. One of the best systems I heard had a Western Electric fieldcoil 555 driver that was run fullrange (not rolled off by a crossover) into a Western Electric 15A horn, with a tweeter filling in the very top of the range and two 18" woofers in a horn-based cabinet. The BIG downside to this system was the size of the speakers--it is like standing two Smart cars on their rear bumper in one's living room.
Another system that makes very good use of a wide-range driver is the Surreal Sound system. I recently heard their active crossover two-way system that utilizes a fieldcoil Lowther driver for the midrange and tweeter. Six woofers provide bass from a separate dedicated amp. This 99 db/w efficient system sounded absolutely fantastic and exhibited very little problems in the way of peaky or rough frequency response. I had previously heard a different system from Surreal Sound that used the fullrange Tangband speaker as the midrange/tweeter. This system sounded very good too, just not as good as the one with the fieldcoil driver.
I have heard a number of fullrange systems that use the Tangband driver. These systems sounded pretty good, particularly given the price of this driver. I bet a system designed around this driver as a midrange driver would be quite price competitive.