Agreed: NS-10s are inaccurate and sound crappy. They are an example of the "other" purpose for studio monitors. They're used for mastering, and their purpose is to give the engineer an idea of what his mix will sound like on a typical medium-crappy consumer system. If you can make your mix sound good on an NS-10, it will sound good on anything.
You'll actually rarely find an NS-10 in a pro mastering studio, they are far more the domain of the tracking/mixing studio. Pro mastering engineers tend to be the audiophiliacs of the music production chain and far prefer the ultra-accuracy of the highest-end far fields.
The whole "if they sound good on NS-10s, they'll sound good on anything" theory is directly related to the midrange bump in their response. (it actually goes even further back than the NS-10; engineers used to love to convert old Auratone public address speakers for use in mix checks with the same rationalization.)
There are/were two ideas idea behind that theory. The first dates back to the days when most folks listened to music on AM radios and crappy portable record players. Producers and engineers wanted to make records that sounded good and sold well on those crappy-sounding (by studio standards) playback systems. So by having a crapping-sounding speaker in the studio on which to check their mixes, they could ensure that the record would sound good in the ol '66 Chevelle or on the ol' Columbia record player.
Nowdays, such crappy playback systems are not the paradigm they once were, but many mix engineers still like to hear accentuation in the midrange area around 3k-5k (give or take a few cycles amongst friends). That range is one of the most troublesome and most "crowded" frequency ranges in the audible spectrum - i.e. most common instruments and voices spend and share a lot of their energy at those frequencies. It is also the frequency range where the average human ear is the most sensitive at virtually all volumes. As such, it is an area that's easy to over-saturate with guitar, keyboard, vocals and such, to the point of easy harshness. By listening on a speaker that harshes the mids easily on it's own, it makes it easy to tone down the mids in the mix so that they then dont harsh on any speaker, including the flatter ones.
Enclosure and crossover design are more-or-less fixed costs. There's no added design cost if you design-in, say, a mid-bass hump. In fact, designing carefully for flat response can consume more design hours than just throwing in a peak somewhere and calling it a day. Crossovers and porting are just some of the ways you can "hype" a design at will without affecting your price point.
True enough. But hyping via crossover design is not what most people are talking about when they talk about "hyping" home speakers. That usually refers to boosting the response efficiency towards the ends of the spectrum in more of a "smiley face" EQ curve simulating something like n A or C weighted "loudness-button-style" boost. And that kind of performance typically requires more expense in the speaker element quality and enclosure or porting design than in simple crossover tuning.
When you wind up with a speaker that is strong in the mids but weak on the ends, that is far more often the result not of "hyping" the mids, but far more simply of not being very efficient or accurate on the ends of the spectrum. That's not "hyping", that's just profitable and economical speaker construction.
And in today's booming "home studio" market, there's no shortage of the desire for marketing highly profitable and economical speakers to take advantage of the market. Nor is the "studio monitor" market immune from the realities of OEM supply and cost, nor the intense competition of having to sell more speakers at better margins at price point X than your competitor. Sometimes the driving force in a speaker design is not some specification goal as it is to whether OEM Smith can commit to manufacturing and shipping 10,000 woofers a month to brand name loudspeaker company Jones fast enough and for a small enough cost so that Jones can get a speaker model on the market fast enough and profitable enough to kill rival company Brown's popular speaker at a similar form factor and price point in time for the next NAMM show.
But beyond all that, the bottom line is that all one has to do to hear for themselves the mythology vs. the truth is to take their ears down to their nearest quality audio outlet, put a blindfold on, and listen for themselves. There is as much difference in studio monitors as there is in home speakers, and there is a whole lot of crossover (pun incidental) area in performance between the two.
And maybe even more to the point (and a driving reason for this wide variety in both camps) is that virtually every set of ears and brains has different preferences. Some people prefer flat, extended, accurate response. Others prefer accentuated [insert your favorite frequency band here]. Some are simply fooled by a speaker design's overall efficiency, and are fooled into thinking that because speaker A delivers a higher SPL than speaker B at the same input wattage, that speaker A sounds better.* Many people - included many pros and self-professed audiophiles - can't even agree on what "flat" actually sounds like. There are those that insist (just for example) that Mackie 824s are "hyped" whereas something similar to the NS-10 is "flat", when in fact just the opposite is far closer to the truth according to every impartial test known to man.
There are a thousand different-sounding studio monitors and a thousand different-sounding home speakers with overalpping performance quality because *there is a market for them*. And there is a market for them because a thousand different people will have a thousand different personal reasons and preferences and taste and hearing differences to create a market for such diversity.
* this efficiency illusion is a popular sales trick used by retail sales outlets across the country. Identify a speaker in your sales inventory that has a relatively high efficiency and profit margin (and therefore commission reward), and make sure it is the "B" speaker in an A/B listening comparison against a less efficient speaker that is perhaps not as profitable. Make a show out of the point that you are not touching or changing any settings on the sound source. 5 times out of 6, the unwitting customer -educated or not - will be convinced that the louder, more efficient (and more profitable for the retailer and salesperson) speaker sounds better, and will likely pick that one to buy - especially if they are at similar enough price points.
G.