I don't think we can dispute the science that proves that random mixing, or placement of bass speakers is likely to result in problems. I believe the effect would be more noticeable at a venue like a large theater, or auditorium that was designed to have amplified sound than it would be in a small bar room not originally designed as a space to listen to amplified music. The small, sometimes odd shaped rooms with unevenly reflected sound waves, combined with an audience of people who are consuming alcohol, who are not critical listeners, and who would not notice uneven levels of bass sound make it not a big deal for the many hobby bands who do mix cabinets, and who also do a lot of other things wrong, (like too loud drummers, and guitar amps, and over-use of nasty sounding effects pedals!) that detracts from the quality of the sound. I help a friend provide sound, who insists on placing subs at each side of the stage, and I have been able to hear the result of comb filtering at large outdoor shows when we have done this by moving around and listening closely. But, I bet nobody else did. I don't gig much, but I would not randomly combine bass cabinets myself.
