Actually, mixing drivers under the right engineering conditions allows for the exploitation of parameters that might be complimentary while avoiding (or designing around) the potential negative effects.
There is so much simplistic "edicts" espoused on this forum with regards to mixing driver sizes that if some folks would take the effort to educate themselves on the engineering behind radiating sound sources and wave theory, they might discover that there are ways to make parameters appear different than their physical attributes might suggest. For example, the whole beaming versus driver size argument so often put forth here... they completely ignore the cone and dust cap geometry and how these can and do alter the mid and higher frequency radiating pattern while APPEARING to defy the rigid rule of thumb so often applied here.
Science and engineering is multi-dimensional, VERY LITTLE is based on single variable functionality because if it did the world would be pretty boring and blah with little need to innovate.