One of the earliest lessons I ever learned about installed sound for worship was the importance of creating a single point source if possible. A single point source located above the speaker makes the sound seem like it is coming from that speaker. This avoids the fatigue associated with hearing the sound come from one place and seeing it should come from another. The other advantage of a single point source is the reduction of comb filtering which can destroy clarity and create other problems. With the Spoken Word PARAMOUNT, shouldn't there always be a single point source for at least that portion of the service? I'm seeing more and more line arrays being flown in LEFT/RIGHT hangs without a center source. I'd like to see a robust discussion of this.
Perhaps we can eliminate the line array factor as it seems your comment really relates more to the single source versus multiple source aspect than to point source versus line array source.
In my experience, most house of worship applications involve some element of both spoken word and music, the latter encompassing playback and/or live reproduction. Therefore developing the optimal overall speaker system concept is typically a matter of balancing both speech and music reproduction as they relate to that church's goals while also considering the budget and the physical space, all of which can vary widely. In some cases there may also be aesthetic considerations that must be taken into account. Thus there is no one 'right' answer, there is instead what may be optimal or practical for each different situation.
I think there are many left/right 'stereo' systems installed because people automatically assume it to be the 'best' option since it's what all the big tours use, what they have at home, what the folks at the music store suggested and what is in many other churches. It's easy to sell and hard to blame someone for doing what so many others do.
I think that many churches get sold on or assume their are inherent advantages to a 'stereo' system but as Ivan noted, having left and right speakers do not make a 'stereo' system. There are two components to this. First, I agree with Ivan that many sources are mono and I think the concept of 'stereo' systems being desirable often has to do with addressing the playback of stereo recordings or the use of multichannel effects rather than reproducing live performances. Second, with large listener areas then unless the system is properly designed the listening experience can vary greatly throughout the listener area and result in very different experiences for different listeners. Many 'stereo' church systems actually provide stereo or even something close to it for a small portion of the audience. I think left/right systems might be less common, or at least less likely to be assumed to be a good option, if more people realized this rather than having the misunderstanding perpetuated.
This topic may get into the difference between selling a system and designing a system. This is not a consultant versus contractor issue, it is the difference between selling something based on what you think the Owner will buy and accept versus developing and offering a solution to their specific needs, goals and situation. As I mentioned above, you can often easily sell a 'stereo' system regardless of whether it is actually stereo or an appropriate solution for the space and use and in many cases may allow someone to do so without having to develop or apply much effort or expertise.
A single point source does have advantages for the spoken word, but those advantages are often over played. A single speaker is easier for person inexperienced person or company to get right. There is less of a chance of throwing too much energy on the walls, and there less to deal with in the interaction of the speakers with each other if it is one speaker.
Sound is one big compromise, and as for a single source point you would often need to sacrifice consistent Db across the room (particularly if it was a wide room or a real large room) or coverage. If the room has a low ceiling single point source system will likely not work.
This may be reflecting the importance of the difference between a physical source and an acoustic source. You will probably not find any speakers used in a church of any size that are a true single point source, most have multiple drivers that are physically separated from one another. However, you design, process and apply them in a manner so that they may effectively function as a single acoustical source to the listeners. You can also take that to the next level and use the locations, arrangements, processing, etc. of multiple speakers to also try to create the perception of a single acoustic source. Thus multiple speakers may be used to try to create the perception of a single acoustic source or multiple acoustic sources.
An example of how this relates to the discussion, having left and right speakers may relate to a system where you are intentionally creating two different acoustic sources, however it could also relate to a 'ecploded mono' system with each speaker covering half of the listener area and trying to create the perception of a single acoustic source. It is not just the lcoations, but the application.
In most real world large rooms and sound systems there is so much comb filtering and reflections that one speaker more or less does not make that much difference.
That may be true for many systems but I think others may disagree about it being inherent or desired in a well designed room and system.