I have a theory on early stereo Id like to share.
I like to think that mixing engineers discovered early on that doing a proper stereo mix and 50/50 blending (in the audio world we call it downmixing or averaging) the audio together resulted in unnaturally loud audio. Vocals are usually panned dead center, in-phase. When you mix the stereo together, your vocals wind up double the amplitude (6dB increase) over the rest of the audio.
So, rather than waste time (and money) making a seperate mono mix, its possible they did the lousy mixing more to avoid the vocal build up more so than to really "show off" stereo, although I'm sure this was another reason.
The reason I say this is Haeco-CSG. This was a simple device that was used HEAVILY by A&M records (as Holtzmann was one of their engineers), but I've also seen it on a Warner/7 artists album. Most specifically, The Association's Greatest Hits. The LP makes note that it employs Haeco. The Cd release doesn't, but it uses the same Haeco'd master tape.
Haeco was an attempt to solve the vocal build up. Engineers could do a proper normal sane stereo mix, and this device would create "compatible stereo" (CSG stood for Compatible Stereo Generator). Its method of operation was simple, electrically shift the phase of a channel. The most common (and recommended) setting was the +3dB build up..which calculates out to 90 degrees.
The idea itself holds up. You shift the phase of a channel by 90 degrees, the offset is enough the vocals partially cancel out and only wind up 3db louder rather than 6db. You could achieve no build up by using a 120 degree shift, but the manual states, and I've verified digitally, if you accidently have your tonearm wired wrong you wind up with something closer to total center cancellation.
The problem was while this was GREAT for mono, the resulting stereo sounded horrible. Center panned stuff was 30% heavy to the right, it sounded like it was partially canceling out inside your head. To make matters worse, apparently some engineers were mixing *through* the device. So...sure, they could make the CSG stereo sound *somewhat* better, if you ever undo the CSG, the resulting panning is off.
The Association is the only album to date I've played with that uses this, at least its one that wasn't remixed from sessions. I've heard evidence where it seems as if they were mixing through CSG; some vocals come out panned more left and often times when the main vocal is trying to harmonize with backing vocals, panned stereo, main voca level will drop dramatically. "Enter The Young" is a track where I noticed this after un-CSGing the album. But that album also features A LOT of bad left/right mixes where nothing was panned center.
Therefore, I've drawn the conclusion that it was a cost/time saving measure for a while. Simply based off that one album. Am I nuts? Ill be the first to admit I am, and I could be way off on this...this is what I've observed from analyzing the audio. It makes sense in my head though.