There seem to be some shocking simplifications in people's responses. A two way will cost less, given equivalent quality components. However a two way places a severe constraint on the low frequency driver .. it must shift enough air to provide bass, but it must also cover the midrange frequencies that are critical for imaging. The advantage of a 3 way system is that the lowest driver can have quite a low roll off frequency and handle the shifting of a lot of air. The mid range (which can now be a smaller cone than in a 2 way design, and can therefore be crossed at a higher frequency with the tweeter) does not have to handle power, not does it have to be crossed in the high mid range, so could theoretically produce a purer midrange. Finally the tweeter comes in at a higher frequency than in a 2 way design, because of the smaller midrange, and so now this opens up the possibility of ribbon tweeters, super-tweeters and the like that give a beautiful and extended HF.
I own a 3 way design (Heybrook Sextet) and a 2 way design (Spica Angelus) and both are great, and different. The imaging is similar on both (very good on both) but the Heybrooks have much more at the frequency extremes. The spicas really need a subwoofer for anything other than chamber music.
I don't think one could say that 2 way design is better than 3 way or vice-versa, and 3 ways don't seem to come into play below $2000 per pair, but I can definitely see advantages of a 3 way design, even if they are more expensive and complex to pull off.
