To say that, "I had a pair of bookshelves, and when I moved to 'Model X' monitors the improvement in sound was huge," says nothing more than your particular model of bookeshelves were not as good sounding as your particular model of monitors.
Your'e right, many bookshelves suck worse than a chest wound. No question. But frankly, so do many so-called "monitors". In my career I have worked with (and played with

) certain models of "consumer" or "entertainment" loudspeakers both floor model and bookshelf from Utah, Infinity, Advent, Klipsch and Bose (and probably others that I forget offhand) that - for the same price or less - would in terms of accuracy of sound reproduction, ease of translation, and rate of ear fatigue absolutely blow away certain models of "monitor" from m-audio, Samson, Alesis, Mackie, Tannoy, and even Genlec (and probably others that I forget offhand).
Anybody that has ever been within a half-mile of a Yamaha NS10 will know just how bad pricey "studio monitors" can actially stink.
I've told this story once before, but it deserves telling again. I had a friend who once set up his recording room with a pair of Advent 5012w's (12" 2'way stand-mounted off the floor) topped by a pair of Advent 2002's (5" or 6" 2-way bookshelves.) He drove these speakers with a Technics consumer integrated amplifier. All middle-of-the-line consumer gear. His room had hardwood floors with plaster walls, but with an extreme mimimum of conventional interior decoration he got the room tamed fairly well. Well, I'm here to tell you that he/we were able to make the best mixes out of this absolutely gorgeous-sounding setup and room made out of not a single piece of "pro" gear in the monitoring chain itself.
I would probably take those old Advent "consumer" speakers over my current Mackie824 nearfields in a heartbeat. Don't get me wrong, I love my 824s, and my room is actually better than his room was, but nearfields just don't have the same feel as room speakers/monitors for me.
And as far as consumer bookshelves alone, I have a pair of Klipsch KG0.5 "bookshelves" haning on wall sconse brackets a few feet above my Mackies. I use the Klipsch as a cross-check/translation source, though not as much as I used to because I know my way around the 824s well enough now to translate without help. I used to have an old pair of Infinity RS10s (back before Infinity went overseas and got crappy) before the Klipsch that were even better than the Klipsch. I'm driving the Klipsch with a Denon consumer integrated amplifier. I'll take those Klipsch ($99ea when I got them in the late 90s) over half of the passive "studio monitors" I've heard for 150% that price.
This thing about "studio monitors" being "flat" or "flatter" than "entertainment speakers" is a load of crap in the econo-monitor price ranges. Some are, yes. Some are not. But here's the point to consider: if these "studio monitors" were so damn flat an accurate, how come no two of them sound the same? There are endless arguments on these boards about m-audio vs. Samson, KRK vs. Yorkville, Wharfdale vs. Tannoy, etc. etc. etc. If there is such a huge difference between brands as to be worth fighting over, there's no "accuarcy" or "truthfullness" in these things. You can't have fifteen different-sounding "truthful" monitors.
So this myth about "studio monitors" being better may apply in general when you get to the big expensive stuff (though I've heard some $4000 "consumer" loudspeakers in my time that would work well in any mastering suite), but in the budget range most often quited by your average home recc'r, the distinction can be more what's printed on the box than what's happening in reality.
G.