Short answer: 1)Frequency response and 2) dynamic response/tone. With technology the way it is it is near impossible to cover the range of frequencies needed for music which are often quoated as 20Hz to 20kHz, or get even very close with a two way speaker. In addition, a two way speaker tends to(not always)push the drivers to their limit in order to cover as much as possible. The result is poorer sound. You will have a tweeter no matter what. In a two way system you are stuck asking the remaining driver to give you nice deep base and clear tight mids. The cone tends to be over worked and breaks up because you are asking it to do so many things (read distortion). Some systems ask the tweeter to do too much too. Tweeters hate that...to much excursion. If the crossover's rolloff rate(this controls how your tweeter shuts down as it receives lower frequencies) isn't steep enough your little tweet will crap out on you. On the other hand if you do not ask your tweeter to go to lower Hz than the burden falls on the poor remaing driver to fight a two front war so to speak.
Most 2 way systems are therefore designed as a comprimise. The middle frequencies (really most essential for listening) are muddied because either the tweeter or remaining driver is trying to handle too much, or the base is sacrificed in order to keep the mid-range better. A 2 and 1/2 way and 3 way system redistributes the load - more hands make lighter work.
That being said, there are some great 2 way systems out there and I have a very nice one myself in my office. They are nice because you can make them so small and in small rooms they fit the bill. (more driver surface, 3 speakers, will move more air and make more sound, sometimes you do not need that either)
IMHO, It is not how many drivers you have but the design and the quality of crossover that is important. I would much rather have a well designed 2 way, with good innerds (great design in the frequency distribution, and quality coils and caps) than a run of the mill 3 way. It is amazing how poor the quality of some crossovers is.
I'd say more but geez... I have to go to work.
Sincerely, I remain
Most 2 way systems are therefore designed as a comprimise. The middle frequencies (really most essential for listening) are muddied because either the tweeter or remaining driver is trying to handle too much, or the base is sacrificed in order to keep the mid-range better. A 2 and 1/2 way and 3 way system redistributes the load - more hands make lighter work.
That being said, there are some great 2 way systems out there and I have a very nice one myself in my office. They are nice because you can make them so small and in small rooms they fit the bill. (more driver surface, 3 speakers, will move more air and make more sound, sometimes you do not need that either)
IMHO, It is not how many drivers you have but the design and the quality of crossover that is important. I would much rather have a well designed 2 way, with good innerds (great design in the frequency distribution, and quality coils and caps) than a run of the mill 3 way. It is amazing how poor the quality of some crossovers is.
I'd say more but geez... I have to go to work.
Sincerely, I remain