Might as well chip in my 2 cents here.

Some good points raised on both sides.

Some points that are blatantly incorrect however, but I'll get to that soon.

First off, there is no such thing as a truly flat speaker. The day someone designs one, they will be a millionaire as a result most likely. Even if such a thing could exist in this present day, it would be prohibitively expensive to put that much R&D money into a speaker that is 100 per cent totally flat, and it would also be pretty much inaccessible to consumers that aren't absolutely rolling in money.

How flat and accurate a speaker system is depends on many variables, such as headroom (which is necessary to produce accurate transients) and indeed how many speakers are in the actual system.

A 2 way speaker near field studio monitor cannot be totally flat, purely as a function of it's design, since they are pretty much limited to a 10 inch low frequency driver. A problem with some monitors made in the 2 way design is the crossover point, which results in some degree of dip in the mids a lot of the time. Some are worse than others in this regard, some are more flat and accurate.

Really, to get a response from one speaker system that is pretty much close to flat, you need 4 way system, with a tweeter, a mid range driver, a mid range woofer and a woofer, and even then, an 18 to 21 inch sub woofer can be used to get an even flatter low end perception.

That said, it's not really a bad thing a totally flat speaker exists, as we have been able to hear from the great results produced from the current available technology.

About room acoustics it's in fact incorrect that a room needs to be totally free of reflections.

You'll find in pro studios, not only absorber panels, at the back of the room there are typically diffuser panels in order to keep the room sounding somewhat alive, since it's more natural anyway.

For mixing, typically a set of great near fields is all that's needed, and in fact is even preferable since nearfields translate better to traditional consumer speakers than main monitors anyways.

For this obvious reason, this means that mix engineers don't have to spend as much on their monitoring as mastering engineers, because near fields are cheaper than main monitors.

It's true many mix engineers often reference with cheap audio systems.

Chris Lord-Alge, the man behind many great mixes like the latest Paramore record, and some of the My Chemical Romance stuff if I'm not mistaken, has been known to reference his mixes using a boombox in mono.

Boomboxes typically have a lot of mid range, but not much in the way of good bass response and poor treble response/detail.

I have a set of cheap computer speakers (2.0 system) at my desk which have that boombox-ish sound quality to them (mostly lots of mids, a pretty poor high end and weak low end).

I've had these for years, well before I started studying audio engineering and I know their sound inside out backwards, so they serve as great reference speakers.

One thing about mixing for higher end systems is, it actually doesn't make things sound atrocious on smaller, cheaper systems.

There is one guy on the Andy Sneap forum who tends to pretty much only mix through his high end Event Opal monitors and has gotten sick of referencing with cheap speakers.

These Opals have a 10 inch driver for the woofer, so you can imagine they have a good, solid bass response and can let you hear a lot of detail in the lower end.

His mixes still come out sounding great on any kind of speaker. 

Sure they might sound a bit thinner than normal mixes on cheaper speakers, but it still sounds good, even on my crappy speakers. Sure, as I said, the sound was a bit thin in the low end, but it was quite obvious that it was a well done done mix with great clarity.

A good mix is a good mix, period.

What it means for his mixes, is that when it gets played through a full range system, like at a club for example, it will sound totally huge and more defined in comparison to mixes that were constantly referenced with cheaper speakers and made to translate to a wider variety of speakers.

As for mastering speakers, well, they are extremely expensive and the closest thing you can get to flat, as I've said.

Personally, I have no plans to get into mastering at all, hence I see no need to buy speakers of that type.

By the time a mix gets send to a mastering engineer anyway, it's already done, so for the mastering engineer it's all about being able to hear all the details so they can do their job