big money PA system do actually use multiple smaller speakers in place of bigger speakers. The reason the majority if the venues we play don't have them is they can't afford them. the nicest Pa I've played through i was told cost over $200,000 and it didn't have a single 18 or 15 yet the lows were insane.
I think the misconception is a lot people don't understand the classes of PA. And there's a bit of false advertising in that department. For example theres low grade DJ systems. This is the type of gear most of use use to amplify vocals at band practice, but the truth is at local venues the systems you may think are "professional" systems are ussually really only higher end DJ level, they are NOT pro level. Most of the online music gear retailers don't even sell real high end PA gear. Mainly because there's just not a huge market. The production companies that do buy those types of systems travel the country with those PAs doing shows and they get paid big money to do it. And those PAs hold up really well and are replaced more so for the next better thing than any other reason.
The reason why can be seen in bass rig changes over the years. Cabs like the SWR Big Ben and accugroove use some of the best big speakers put in bass cabs. They were not the norm. Most 18 cabs lacked range and were slow. Not only do most 18s not go any lower than a lot of modern 10s and 12s but they typically lack a ton of midrange 10s and 12s do have. Now in certain types of music, for example hip hop where the bass is mainly sub and there's not much demand for mids 18s work fine. But for bass guitar to really sound like a bass guitar and have a full range sound to include deep lows and enough mids and highs that the bass sounds like a bass guitar and not a pulsating sine wave with a little attack. 18s just typically don't cut it. For me it's obvious when I hear PA systems particularly with hard rock and metal bands because difference in the richness of guitars and the bass is night and day.
I do have to say that I do think 18s look cool in cabs where you can see the speaker. And for biamping they can often be used very effectively but most of us who gig a lot don't want to deal with bi amp rigs. I'm not saying they don't work or don't sound good but they typically are a pain in the but. Not only because they require multiple cabs, but because you've gotta practically bribe the sound guy with beer to give you multiple channels of bass.
