Are you sure of the crossover points they are using? The horn can cover quite a bit of the treble and midrange.
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15 inch won't go more than about 500 Hz before beaming, however, horns are narrow directivity anyway. Done correctly, they will try to match the beaming from the 15" driver with the narrow directivity of the horn to make a smooth transition in the off axis response. Without a third driver it may end up with some upper frequency roll off but it can be made to work with careful design.
Assuming a high quality woofer and good crossover design, a 1000 Hz crossover is appropriate for a 15" woofer and 90 degree horn. Altec used a 1200 Hz crossover for the 15" woofer and horn in their epic Model 19, which was said to be a significant improvement over its predecessor the Valencia (800 Hz crossover). The rationale behind the higher crossover point is as Shadorne said: Matching up the radiation patterns. In my opinion it's worth doing, and the configurations Tyler is using in their Pro Dynamics line are well thought-out.