Often a speaker designed for mixing is missing the bottom octave and a little less sensitive, where bass problems and dynamics are often the bread and butter of a mastering dude. Conversely most speakers "made for" mastering are a little more sensitive/fragile and a bit bulky/needy in the room placement department. I've certainly done more than a couple mixes on my Dunlavy's, and I have friends mixing on ATC's, PMC's and B&W's. On the flip side seeing as most mix rooms are also tracking rooms, and clients often are listening back, changing levels etc. a mix monitor tends to be more robust than a mastering monitor. I've worked my mastering career mostly with 802's and Dunlavy's and never blown a tweeter. But bring up a rock mix, with a careless client at the helm, or track something quiet when the client accidentally hits the mic and you'll likely have some pricy regular maintenance/driver replacements going on.
So basically, in a mastering setup headroom/bass extension are more the concern, whereas with a mix/recording setup you can pretty much drop the bottom octave, and give up a little headroom for something a bit more robust mechanically. Can a speaker do both? Of course, but that's probably gonna cost you. Also I know a lot more mixers that work in different room based on budget, and travel with their speakers... Now imagine the same with a set of Duntech Princesses, or BB5's, well geez now a pair of Studio 100's or 1031's are sounding great. There's a million reasons why there's a million different speakers out there, it doesn't make one worse or better, (of course there are things that do!) but you need to use what best gets the job done, and meets the requirements you need in your particular situation.
why isn't the monitoring equally important in BOTH areas? In other words, how can a speaker be "good enough for mixing but not for mastering"?
It seems a lot has to do with the room design. Mix rooms these days typically aren't designed and built to support the room for floor standing far field full range speakers that are popular for mastering.
Ideally, I thinks it's easier to mix in a purpose built mastering room than it would be to master in a purpose built mix room because of the speaker proximity alone, although I've seen hybrid rooms that are suited for mid fields which can even out the playing field a bit..
Actually bought a set of (Tyler, of course) D4M's last year (A) for a "smallie" reference and (B) for the occasional mixing project (doesn't happen much, but I have a few clients from "back in the day" that still want me to mix).
I have to admit, that I mix faster on the small speakers -- Probably because I'm not sweating every little detail to get it all together. THAT SAID, once I'm "in the pocket" I switch over to the D1's and start the hardcore tweaking...
^ there is a reason why Yamaha NS-10m have been used for mixing the world over. Music production and i believe mixing is part of this, the fine micro detail world of high-end speakers tires the mind and sometimes links the musical ideas to irrelevant splitting of atoms and creates obsessive attention to datail which sometimes backfires at the big picture. As in fine arts, one cant constantly use micro brushes to paint a 20x30 canvas, its tiring and places too much responsibility on the artist. And sometimes in music its simply, what is wrong often sounds right . Also because of the fact that todays cheaper converters output fidelity 10X what tascam portastudio's ,lol , or other recording mediums did back then, working on 100x magnification is once again not necessary for the majority of todays playback systems. And ITB(edm, etc) generated material does not suffer from cable noise, multi-mic phase issues, etc or other inconvenience of acoustic music, less need for extreme magnification. The best answer ofcourse is to have both, detail monitors and ones that are funke
It's like comparing a range-finder to a ground glass viewfinder.
Speed often leads to much better mixes so a range-finder can be a really useful mixing tool. Mastering is about catching minutia that may have been missed by the range-finder so the ground glass seems more appropriate.