Using only one full-range driver in a speaker gets as close as possible to an ideal point-like sound source. It emits the entire audible frequency spectrum, and unlike speakers with several drivers, doesn’t need shunts and can be connected directly to the amp.
For multi-way speaker systems, however, the input signal must be split in the different frequency ranges, which requires significant technical efforts, bringing about a number of severe problems:
You get phase displacements. A single sonic event, that should reach the ear at a distinct point of time, is broken up into a number of discrete events, thus impairing the precision of sound reproduction.
Directional hearing requires a certain interaural time variance. If this variance is ambiguous, however, depth information and plasticity get lost. Formants are frequency ranges containing particularly strong harmonics. They have great influence on the timbre of a sound. When formants are distributed over more than one driver, the characteristic spectra of instruments and human voices are impaired.
Psychoacoustics teaches us about the recognition of instruments depending greatly from its temporal flow, especially attack and decay. If the reproduction of these events is not correct, characteristic features of instruments are mangled. The consequence: the sound seems sterile.
Full-range drivers eliminate all of these problems. The resulting homogeneity and plasticity are superior to everything you could expect from multi-way speakers. The character of the performance remains unadulterated. The music sounds deceptively real. The atmosphere of the room where the music was recorded, be it a great concert hall or an intimate jazz club, is reproduced amazingly well. You’ll feel like actually being there, not like listening to a sound conserve.