My experiences thus far tell me that each design has strengths & weaknesses. For example: a good PURe single driver setup (with no passive networks & no other drivers) has a purity and simplicity and coherence that is hard to match...even one uses cheap drivers like the Fostex FE206E. On the other hand, one will lose frequency extension and flatness. A 2 or 3 driver setup with minimal passive networks can take care of these problems (as i have done in my trusonic + tweeter + BSC setup) but at the expense of some of that coherence and imaging.

I think we have to look at the driver & decide...for example i have read about (but never heard) of mongo rear loaded horn cabinet designs for Lowthers that claim to give great coherence, decent imaging and good frequency extension (flatness may not be that good with Lowthers). In the case of the trusonic 80FR, it is clearly not great for a PURe single driver setup, however, it does pretty well with a tweeter and a BSC.
-akhilesh