Allow me to semi disagree with a bunch of what you've said.
There are quite a few speakers that, while not super narrow, are certainly not very wide either (medium dispersion?) that get a whole lot of love here--lots of Kefs, larger Genelecs, Neumanns, various JBL horn speakers, etc. It's not just a question of narrow vs wide, it's good vs bad. A really good narrow dispersion speaker can still sound quite good outside it's -6 dB rated/tested beamwidth, providing a smooth and flat response that's just down a few dB (which actually may be desirable for particular applications as I'll describe below).
The problem with simple horizontal MTMs such as this is not just that they're narrow. It's that they're a
bad narrow. Listeners much off axis get a really lousy direct sound. In my experience, yes, this is very audible. I'm not aware of any science or tests or even opinions of scientists or testers that identify this as anything other than being a really bad thing, agreeing with what my ears tell me.
I would also add to that (especially anywhere near this price range) multi-purpose rooms and living rooms. While having everybody "in the narrow cone" is certainly ideal, most people simply can't afford to waste that much space. My favorite chair at my parent's house is about 60 degrees off axis... Even for home theaters, I think a great many people wish they had bigger rooms but they're making the best of what they have. Often the front rows are maybe a bit closer than they should be, but I don't think I've ever even seen a picture of a front row of an actual theater build with only one chair.
The goal of a multichannel system should be to provide the best experience to everybody you can (or would like to) fit in a given room. Having to toss out a number of seats because your speaker choice gives those seats such a terrible experience is not the right way to go about it.
That's certainly true, but there's no need to add to that problem and make it worse with a really bad center channel. It's kind of like saying if you have any major health issues, you shouldn't bother going to the doctor if you break your leg--you're already screwed, so just lay there and suffer! A good center channel isn't going to fix all the problems for you, but it will give you a better experience than a bad one will.
As for giving that listener an even better experience, that really goes beyond discussions of the center channel. But since I alluded to possible advantages of a more narrow dispersion for some applications, I'll explain.
While in my opinion, dispersion width for two channel listening is largely a matter of taste/personal preference, to me for multichannel systems intended for multiple listeners it's much more of a math/geometry problem. Given a room size, seating locations, speaker locations...cover the seats in good sound. Going much beyond that may do more harm than good.
Considering the case of that chump stuck sitting in the front row right in front of the left front speaker...if that speaker has extremely wide horizontal dispersion, even if he's 40 degrees or so off axis, he's certainly going to get fantastic sound from it. But it's going to be loud, very loud. It will tend to drown out or mask the sound of the center channel (biggest problem) no matter how good that center channel is as well as masking all the other channels.
But if that speaker has a more moderate or narrow (depending upon room size, distance, etc) horizontal dispersion and is a really good design, and is toed in fully to the MLP, he'll still get a nice flat, smooth sound from it and it will sound very good. But it will be down 6 or 8 dB, depending upon the exact dimensions, dispersion, etc. This really helps his ability to hear the other channels.
Will it turn his seat into an ideal seat? Of course not. But it will give him a better "multi-channel" experience instead of a "left front channel only" experience.
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