"If a crossover is designed correctly, then shouldn't the resulting response be flat? If so, then why the whole deal about full-range drivers? "
Full range speakers is generally a niche, just like horns, esls, etc. As with everything, each type has to compromise.
As MreHost stated, the main claim that FR enthusiast assert is the lack of a crossover. Well, this is a debatable topic that could go on forever between parties. As well as lack of a crossover in the sensitive range, also phase response is 'generally' bettter on a FR. However, this is not true when considering multi-way designs using true transiet perfect 1st order acoustical slope crossovers, which have very simalar phase response. And, most FR drivers have worse phase response than properly designed TP multiways. Of course, even the audibility of phase distortions of high order xovers(2nd order and greater) is debatable. LOL.
Dynamic FRs offer some very substantial negative attributes though: high intermodular distortion and poor power response, through the intended bandwidth. One thing I do not personally agree with MreHost about is ESL vs. other designs. Thier are applications using converntional cone drivers/planars/hybrids etc that have very similar transient decay characteristics to an ESL(the main benefit of an esl), but with BETTER power response(off axis response). I do admit, however, that these are rare for numerous reasons. So, in GENERAL....ESLs offer better 'clarity' than other designs. MreHost probably meant MOST, but so many are inferior, ONLY considering spectral decay, that this would be an accurate general statement. As far as crossovers being located in the sensitive ranges, this is avoidable with very careful and selective design. But the added costs of achieving this are usually prohibitive, ESPECIALLY in retail products, and substantial skill is required to acheive this in the DIY route. Also, keep in mind that EVERY paramter'w importance is dependant on the intended end user's preferences. Many people specifically prefer speakers with worse spectral decay response, as this adds sometimes 'life' and 'texture' to the sound, which in reality is coloration. In this case, the last speaker such an individual would want is an ESL or transducer system with similar spectral decay behaviour. Also, such speakers are ruthless, requiring exceptional recordings..if you wish to achieve a virtual performance in your listening room. In this aspect, I don't know a single dynamic FR driver that exhibits very good spectral decay response. This is reserved for ESLs and Mangers, in the FR neighborhood. But as I stated, this is a preference. You should sample all types, and decide your preferences. That is what is really important anyways. Targeting ideal measurements alone will lead you to a DEAD END.
-Chris