This statement is completely subjective and completely untrue. I've seen a bunch of installation pics with 2 or 3 rotary subs used in a modest living room type setup. For churches and other commercial usage I've seen many more used - the most I could find in a quick search was 6 but I've seen pics with even more used.
There's no doubt that the rotary sub is impressive but it could easily be beat by conventional drivers (in multiples) given the same budget and size limitations.
The number of subs required (rotary or conventional) has a lot to do with the room (construction, leakage and size), the desired spl and the available budget.
Here's some pics of multiple rotary subs. One domestic, one church, one commercial. (The commercial pic shows 6 subs, link to the writeup is here -
Niagara Falls Fury rotary woofer installation )
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Those installations, except one are commercial
installations and are huge in size requiring a lot
of subs.
Mine room is 20' wide x 30' long and I use
less than the full output of the rotary sub.
Transient response with rotary sub (below 10 hz
is one of the hardest things to get right, and the
rotary excels in this area.
I saw one installation with (eight 18" subwoofers)
with 10,000 watts of power that could equal
the rotary and surpass the output, but I am
talking about one rotary sub. No single
conventional sub can produce enough energy
below 10 hertz that can do what the rotary can
achieve, especially with only 200 watts!!
If one has the room for this many drivers,
and enough power, this would work well,
if loudness is one's only criteria.
Distortion is low with the rotary sub and
is a perfect match for my Magnepanar Tympani
IV's (coherence is superb with no apparent
colorations, which one would get with conventional
subs.