As far as flat frequency responses go, I think there is a difference of perception and semantics of what that exactly entails. Some folks mean flat as in a horizontal line, others mean flat as in a straight line, BUT it may not necessarily be horizontal in-room.
I DO want a speaker that can measure flat anechoically. I wouldn't want anything with inherent peaks and dips in the design because speakers with built-in uneven responses like that can be practically impossible to correct. They will skew the sound of whatever is playing. Some folks may like the skewed sound, but that would be up to individual tastes. I don't think speakers, amps (or cables) should act as tone controls. I want all those components to have as little variation and coloration as possible.
Now that article has a point. A measured flat in-room response is not perceptually flat. The Harman studies have shown that people tend to perceive a falling response as flat, and their tests indicate that the most preferable frequency response is one that is as smooth as possible with no major peaks or dips, and gently decreases as frequency increases. It's probably because we're accustomed to hearing that inside any structure with walls.
As it turns out however, because room gain tends to have a greater effect on the lower octaves, and the higher frequencies experience greater decay over distance, a speaker with an anechoically flat FR tends to naturally exhibit a falling power response when placed in average sized rooms and measured at average listening distances. With the exception of constructive and destructive interference causing peaks and nulls (which can be greatly reduced with placement and acoustic treatment), the natural in-room response tends to emulate the 'Harman curve', just as my speakers do in my listening room. A little EQ'ing and I get a nice, smooth FR.
A speaker incapable of measuring flat anechoically is probably not going to have a smooth FR in-room either, and unlike the flat speaker, treatments and EQ are probably never going to get the FR smooth. I would much rather start off with a speaker that I KNOW is capable of a smooth response and fine tune it in-room, than a speaker with a response that needs to be fixed even before I begin.
Of course, there are other factors like the off-axis response, bipole or dipole design etc. which will have differing effects and cater to different preferences, but to me, the ability to produce a measured smooth/flat FR is part of the basic requirement for consideration. I don't exactly want a speaker that produces a smooth but sloped decreasing response anechoically either, because its in-room response is very likely going to produce an even steeper slope.